Max oil temp

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andyb
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Max oil temp

Postby andyb » Tue Aug 07, 2018 9:49 pm

What is the max safe oil temp measured in the sump?
I now have a dipstick oil temp gauge. Calibration is good at 100C (boiling water) and it has a red line at 120C, which may or may not be a realistic maximum.

I use Rotax 912 engines which have a max oil temp of 140C and a normal operating temp of 90 - 110C. This is which semi-synthetic oil, not fully synthetic, just like my R100R....so I guess 120 - 130C is a sensible max airhead oil temp?

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Re: Max oil temp

Postby barryh » Tue Aug 07, 2018 10:39 pm

I think there are two questions:

What is the safe maximum temperature for the oil before it oxidizes and breaks down.

What is the maximum safe oil temperature for adequate lubrication of the engine.

I think you can dismiss the first as modern oil will handle 140 C and still last for a decent oil change interval.

The second question is the one to worry about. Once past 100 C the oil is getting thinner than the original design intent. A way of illustrating this is to say that at 140 C the viscosity of 20W50 oil will have dropped from a typical 18 cSt at 100 C to approx. 7 cSt which is thinner than a 0W20 oil would be at 100 C and who would run an airhead on 0W20 even in cooler climates. Another thing to bear in mind is the oil in the bearings will be hotter than the oil in the sump. In practical terms it means the oil film thickness in the bearing gets thinner with increasing temperature but how thin is too thin ? Airheads seem to survive high oil temperatures but that doesn't make it an ideal situation. I'd want to fit an oil cooler if sump temperatures regularly reached 120 C.

If you think 120 C is not too high lets do that viscosity comparison again. at 120 C the viscosity of 20W50 oil will be approx. 11.7 cSt which is like running an airhead on a 0W30 oil at 100 C.

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Re: Max oil temp

Postby boxerman » Tue Aug 07, 2018 11:17 pm

I have no idea of the max temp but the simple facts are that for normal usage the correct rated 20/50 mineral oil will work just as intended unless you're stuck in a traffic jam in 35-40C temps in which case the slight rattle from the engine will let you know that you need to let it cool down a little.
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Re: Max oil temp

Postby Roy Gavin » Wed Aug 08, 2018 8:28 am

Oxidation also increases dramatically as the oil temp increases past 100C- my F650 GS oil ran at 120C/130c and Castrol's finest was cooked in 500 KM.
That was back in 2003 - I think Castrol oils have improved a little since, but up was the only way they could go back then.
FWIW I have to wring my Funduro's neck to get the oil past 90C.
Fitting a oil pressure gauge to an airhead is cheap and easy,and as barryh says, when you see how quickly the pressure goes down as the temp increases you will appreciate why the red line starts at 120C.
After lubrication, oil's second most important task is to cool the high stress areas that are not cooled by the head/barrel fins, and it it probably does this better if the oil is cooler than the part it is trying to cool, which might be another reason some folks like to keep the oil at the temp that the engine designer intended, which is usually 100C.
But of course if you run a G/S off road in Oz on days where the forecast temp is well over 40C oil temp is probably more of a concern than if you only have to contend with a Scottish summer, getting it heated up is probably more of a problem over there unless you have a fault somewhere or keep the motor running too long when the bike is not moving.

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Re: Max oil temp

Postby barryh » Wed Aug 08, 2018 9:01 am

I've just noticed andyb has a Guzzi V7. I thought they had a reputation for high oil temperature more than airheads and as a result isn't the recommended grade a a 10W60. If there was a concern about high oil temperature on an airhead then the same 10W60 would be an option.

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Re: Max oil temp

Postby andyb » Wed Aug 08, 2018 9:08 am

Where have you fitted an oil pressure gauge? Electrical or mechanical?
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Re: Max oil temp

Postby Rob Frankhamr » Wed Aug 08, 2018 9:52 am

I think it's very easy to over think this one. Having a gauge just makes one worry about a problem that doesn't really exist, not in the UK anyway,

For some time, I had an oil temperature gauge fitted to my R100Rs. Not a commercial mechanical gauge but an industrial electronic gauge which measured digitally to 1/10th of a degree and was gauranteed accurate at that level. I appreciate that the rs has an oil cooler but this very seldom comes into operation. During the time I had the gauge fitted, there was only one occasion when the gauge read over about 101 degrees and that was when I made the cardinal mistake of riding into Brighton on the hottest day of the year where I became immovably stuck in traffic. Even then, the engine started to display obvious signs of overheating before the oil temperature got near 120 degrees. It's also pertinent to mention that, on that same day, once I got clear of the log jam of traffic (not by riding on the pavement at all your worships... now would I do such a thing... the very idea! :roll: ) the temperature almost immediately dropped back to normal. Based on this, I think that the chances of overheating the oil in an airhead engine under normal usage is very small. I stripped the gauge off eventually, it wasn't telling me anything I didn't know and I got bored with it. :sad:

As an aside, one thing I did learn from this gauge, and which rather surprised me, is that, in normal use, you need to ride around 10 miles before the oil in the sump reaches a constant working temperature.

The idea that the oil supply to the rockers has any marked effect on head temperatures has been around for years. I'm afraid that I don't give it much credence. The amount of oil supplied to the rockers is very small... Needle rollers don't need a copious supply of oil and oil to the rockers is metered at the Crankshaft to limit the flow. In addition, the oil is not directed towards the exhaust valve guides where it would do most good. My take is that the amount of oil used would have no significant effect on the head temperature.

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Re: Max oil temp

Postby Mjolinor » Wed Aug 08, 2018 10:01 am

Not at all BMW specific but I made this is 2012 to measure oil temperature on my Smart car. It has been very useful lots of times since:
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Re: Max oil temp

Postby Mjolinor » Wed Aug 08, 2018 10:03 am

This is oil temperature against time from cold, ignore the loss of data at just after 10 am.
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Re: Max oil temp

Postby andyb » Wed Aug 08, 2018 3:57 pm

Hi Barry,
You are correct about the high oil temperatures on the V7 and the use of a 10/60 oil. I don’t think the V7 needs ZDDP in the oil as the BMW airheads do, as it is a much more modern engine with a CAT in the standard exhaust system, so uses a fully synthetic. It also only takes 2L of engine oil.
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Re: Max oil temp

Postby barryh » Wed Aug 08, 2018 5:26 pm


It is the bike that BMW should be making!
I've long admired the V7 Andy. If I was to buy a new bike that would be it and I think I'd find the light weight very attractive in my old age !

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Re: Max oil temp

Postby Roy Gavin » Thu Aug 09, 2018 3:24 pm

Andy, on my G/S I have a matching pair OE style pods with a rev counter one side and oil temp gauge the other.
The indicator bracket on the G/S has provision for fixing them, but a bit of flat strip steel out from one of the handlebar clamp bolts would probably do to.
There are pattern pods around with either central or offset fixings depending on the gauge you choose.
VDO have a compact sender with twin terminals for the warning stock light and electric gauge, the 15 psi one I picked was 1/4 npt and needed an adapter to the 12mm? pressure switch hole.
All looks neat and factory when it is fitted.
I have tried the Motul Guzzi 10/60 as my preferred cafe/ garage was also a Guzzi dealer, but surprisingly once the oil is warmed up the oil pressure isnt much, if any, different to the Mobil 1 20/50 V twin Harley oil I normally use.
FWIW it takes me around 3 to 5 km for the oil to drop down from a cold 120 to its normal 60 PSI at 3000 revs, depending on the ambient temperature.
It was 3 C this morning when I left and it might even been a little more than 5km ----------'.
Picked up to a pleasant 20C late, but still midwinter here.
My temp gauge is a dipstick type so I just about have to pull over to read it, but as the pressure gauge gives an instant indication of increased oil temp it is really superfluous.
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Re: Max oil temp

Postby barryh » Thu Aug 09, 2018 4:51 pm


I have tried the Motul Guzzi 10/60 as my preferred cafe/ garage was also a Guzzi dealer, but surprisingly once the oil is warmed up the oil pressure isnt much, if any, different to the Mobil 1 20/50 V twin Harley oil I normally use.
At the very great risk of making this an even more technical oil thread it's perfectly possible that there wouldn't be much difference in oil pressure between a 20W50 and a 20W60.

The 50 and the 60 represent the SAE viscosity bands that the oil falls into at 100 Deg C so you could have an oil at the top of the 50 band and an oil at the bottom of the 60 band and there wouldn't be much difference between them. That's the first point but it's not the real issue.

The main point is that oil pressure is not directly related to the nominal viscosity. Oil pressure is determined by the viscosity of the oil in the crankshaft bearings and the best measure of that is the High Temperature High Shear (HTHS) viscosity which is tested at 150 Deg C being more representative of the real temperature in the bearings. You can have two oils that are nominally the same, say both 20W50 and they might have different HTHS figures and so produce different oil pressures. So fundamental is this issue that some oil professionals would like to see oil marketed by the HTHS figure. That's not likely to happen but if you want to run an oil that will keep protecting the bearings at very high temperatures then the highest HTHS figure is something to look out for.

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Re: Max oil temp

Postby Galactic Greyhound » Thu Aug 09, 2018 6:09 pm

Great stuff Barry - one learns a lot on this forum! :grin:
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Re: Max oil temp

Postby andyb » Thu Aug 09, 2018 7:19 pm

Hi Roy,
Thanks for the info on the oil pressure gauge - I guess you mean a 150psi sender? I did not realise the oil pressure was so high in these engines.

In a previous post you said that the engines are designed to run with an oil temperature of 100C. Just wondering where that came from? I would have thought the ‘ideal’ temperature would be a little higher to evaporate water out of the oil. More like 105 / 110C.

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Re: Max oil temp

Postby barryh » Thu Aug 09, 2018 8:30 pm

That's a good question but I doubt there is a perfect answer not least because there is no single oil temperature in any engine. There will be a range of temperatures from higher in the bearings to lower in the sump. In water cooled engines sump temperature will tend to be close to coolant temperature but a little higher so with coolant typically around 90 C then sump oil temperatures of 100 Deg C sounds reasonable. With air cooled engines oil temperatures are all over the place from cold in the winter to hot in the summer and there is nothing driving the oil to any fixed temperature. Some will never achieve sump temperatures quite as high as 100 C but water is still driven off.

If the 100 C benchmark comes from anywhere it might be the SAE J300 standard which is where your 20W50 designation comes from. In that standard the viscosity of oil is specified at 100 Deg C.

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Re: Max oil temp

Postby boxerman » Thu Aug 09, 2018 8:55 pm

I guess you mean a 150psi sender? I did not realise the oil pressure was so high in these engines.
The oil pressure relief valve opens at 70psi but on a cold engine you'll often see 120psi with a few revs
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Re: Max oil temp

Postby Roy Gavin » Fri Aug 10, 2018 12:46 am

15 psi is the warning light setting, the gauge is 150.
BMWs warning light setting is much less, 5,6,7 depending on who you ask and as the gauge is only accurate to +- 2 psi it may not give much warning.
The 20/50 Mobil one V Twin oil I use is at the highest point possible for a 20/50, measured in centistrokes at 100C, but HTHS figures are hard to find for most oils.
I was surprised only by the fact that a recently introduced full ester synth top line product designed specifically for hot running air cooled twins like Dukes and Guzzis and recommended by them as soon as it was introduced seemed no advance on what I had been using for years.
Havent tried the Motorex/KTM 10/60 yet, though.

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Re: Max oil temp

Postby Rob Frankhamr » Fri Aug 10, 2018 9:36 am

The question I have to ask is...

If an airhead engine can run perfectly satisfactorily for hundreds of thousands of miles (or kilometers even) on full mineral oil (as they can) and assuming that using other oil doesn't affect the service interval (which it doesn't) and accepting that it doesn't make a great deal of difference to performance (which I've never found any reliable evidence to suggest it does). why the heck would we spend several times as much money every oil change...?

Or is there something I'm missing?

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Re: Max oil temp

Postby Roy Gavin » Fri Aug 10, 2018 12:53 pm

In Oz there isnt a lot of difference in price, probably the price of a couple of pints a change.
Ester synth is simply a better product , there are UOAs around on BISTOG showing nil FE after 10,000 miles, something you will never see with a dino.
That is, no wear on steel components whatsoever, and I for one think that is worth having
Simply put,whatever performance you get from a dino you will always get better with a synth, wear, high temp stability, everything is just a step up.
Also BMWs current recommendation if that you use some sort of synth, and not everyone suffers from the delusion that they know more than the factory and the oil manufacturers, and are happy to do what the book tells them to do.
But it is like everything else in motorcycling, some folk want the best crash helmet and riding gear, the best tires, the best suspension and brake components, the best battery and oils, etc, even although lesser items would probably suffice for most of the time.
But if/when the day comes when they need the extra performance/ protection that the best products give they are going to be very glad that the have it.
Cant think of any reason why anyone would want to exclude oil from that list, you may just as well rage about folks using a better class of crash helmet or whatever, but few ever do.
The bikes use might come into it too, bush bashing in second gear on a G/S on 45C days might conceivably give the motor and oil a harder time than a careful ride on a wet and miserable 45F day where the best you can hope for is that the rain will blow over and the wet road will turn to black ice!
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Re: Max oil temp

Postby Rob Frankhamr » Fri Aug 10, 2018 5:53 pm

Hi Roy,

I would say that I suffer from very few delusions and I don't 'rage' at anything, I simply ask questions. Sadly, you're reply doesn't address the point I made.

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Re: Max oil temp

Postby windmill john » Fri Aug 10, 2018 7:25 pm

I do think some of the latest threads are going too deep into certain subjects.
I understand if you keep your bike for at least 200 to 300,000 miles you might get anxious.
I got so stressed out choosing oils, I now stick with a ‘decent’ one. Yes it costs me 30 plus pounds for 4 litres and I sometimes get jealous when someone is using a £12 version, but at least I’m not stressed anymore.

Oh, don’t tell anyone, but I do let my oil go past 6000 miles between changes.... close to, but not less.
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Re: Max oil temp

Postby andyb » Sat Aug 11, 2018 8:46 am

Well this was not supposed to be an oil thread, and it has gone a bit off track,so sorry about that John.

Still looking for information on what the max oil temperature in the sump should be? Any BMW recommendations?

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Re: Max oil temp

Postby CaptAirhead » Sat Aug 11, 2018 9:55 am

I looked at this a while back and read lots of stuff on different forums and still haven't got a clear answer. Same thing with the question of oil pressure. Problem was that owners living in different countries with different weather patterns, using different models and using different oil all seemed to have different answers.
I've heard several stories about owners buying the dipstick temp gauge and then freaking out when it peaked at between 120 and 140C. I actually have a temp gauge and sender which I was going to fit to the sump plug ( the deep sump with the rear facing plug). The gauge reads up to 150C.
I'll get round to fitting it sometime but it's not exactly a priority, especially in our climate.
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Re: Max oil temp

Postby george baker » Sat Aug 11, 2018 10:31 am

Hi
-- owners buying the dipstick temp gauge and then freaking out when it peaked --

Which made me decide not to buy one, so I have one fewer thing to worry about

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