Best way is to check the temp of the oil - what you need to measure is the peak temp when the engine is being run as hard as you normally run it.
I was pushing 300* on my air cooled 250 - now I never exceed 250 on my liquid cooled engine.
The engine temp will vary greatly depending on where you measure the temp. On the clutch cover about 200* - on the bottom of the engine case about 220* - on top of the engine case about 250* - the cylinder about 275* - the intake side of the head about 300* - the exhaust side of the head about 500* - the top of the cam cover about 300* - the head at the base of the spark plug as high as 700* (or higher). Regardless of the parts of the engine, you don't want the oil to exceed about 275* - most oil starts to breakdown at 300*.
Here is the lab notes from one tester testing just 12 brands/weights of automotive engine oils:
"The official test for this is called the NOACK Volatility Test. In this test, the oil is heated to 302* F for one hour. The lighter oil fractions will vaporize, leaving thicker and heavier oil, contributing to poor circulation, reduced fuel economy, increased oil consumption, increased wear and increased emissions. The test reports results in the percentage, by weight, lost due to "volatilization."
Before July 1, 2001, 5W-30 motor oil in the United States could lose up to 22 percent of its weight and still be regarded as "passable." Now, with GF-4, the maximum NOACK volatility for API licensing is 15 percent. European standards limit high quality oils to a maximum of 13 percent loss.
Here are the approximate observed temperatures at which the various oils started to vaporize/smoke, which indicated the onset of thermal breakdown:
5W30 Pennzoil Ultra, API SM = 280*
5W30 Mobil 1, API SN = 265*
10W30 Lucas Racing Only = 290*
5W50 Motorcraft, API SN = 275*
10W30 Amsoil Z-Rod Oil = 300*, the BEST in this test
20W50 Castrol GTX, API SN = 275*
5W20 Castrol Edge w/Titanium, API SN = 280*
10W30 Joe Gibbs XP3 NASCAR Racing Oil = 280*
5W30 Castrol GTX, API SN = 280*
10W30 Valvoline VR1 Racing Oil, silver bottle = 260*, the WORST in this test
0W30 Brad Penn, Penn Grade 1 = 280*
5W30 Royal Purple XPR = 285*
Here are the “averages” for the onset of thermal breakdown with these 12 oils:
Full synthetic oils = 282*
Semi-synthetic oil = 280*
Conventional dino oils = 272*
These observations are perfectly consistent with the NOACK Volatility Test that is performed at 302*F. Oils have to be vaporizing/smoking by 300* in order to perform this official test. For the oils tested above, certain specific oils did show a significant difference, such as the synthetic Amsoil Z-Rod oil which had a 40* advantage over the conventional Valvoline VR1 Racing Oil.
But, as for overall averages, there was only a 10* difference between synthetic and conventional oils. So, the real world observation here does NOT support common internet oil info claims about synthetic oils having an unbelievably high temperature capability compared to conventional oil.
Don’t believe everything you read on the internet about motor oil. Because there is a lot of misinformation floating around, that has often been repeated over and over. Most sources never ever do any independent testing at all, they just repeat what others have already written. And it doesn’t matter how many times, different sources repeat the same wrong information, it will never magically become true.
Performing real world “wear testing” is only way to determine the true story about which oils actually do provide the best wear protection. And this is precisely why I decided to perform my own testing. That way I could see for myself what is real and what is not.
The above info also makes a good case for running an effective oil cooler setup, if one is needed to keep the oil safely below the threshold of thermal breakdown. But you may also need an oil cooler thermostat as part of that type of setup as well, so that the oil doesn’t end up too cool. You should keep oil temps above 212*F to keep the normal engine condensation quickly boiled off, rather than just slowly evaporated off. You don’t want to allow slowly evaporating water to have the chance to mix in with the oil and dilute it. Oil can only be thinned out by becoming diluted with coolant/water or fuel. And oil can only get thicker by getting overheated and vaporizing its lighter components. So, an ideal temperature range for most motor oils in general, would be between 220*F and 250*F. You get the idea, not too cold, not too hot, just right.
I did not test the cold flow capability of synthetic oils here. So, that claim’s validity remains to be seen. But I did perform one last test here, and that was testing at 325*F, to see what wear protection capability still exists during extreme heating conditions. I selected the highest ranked low zinc oil, 5W30 Pennzoil Ultra, API SM and the highest ranked high zinc oil, 10W30 Lucas Racing Only. Even though they were both vaporizing/smoking a lot at this temperature, here are the results at 325*F:
1. 5W30 Pennzoil Ultra, API SM
98,329 psi “load carrying capacity” (essentially no change from its 275* value)
2. 10W30 Lucas Racing Only
97,561 psi “load carrying capacity” (essentially no change from its 275* value)
As you can see, their load carrying capacity leveled off and stayed approximately the same between 275* and 325*. So, it is comforting to know that you don’t run into dangerously low wear protection if and when you end up with overheated oil at some point. But of course the oil will have already run into thermal breakdown and should be changed as soon as possible."
This tester only tested API rated oils and did not test JASO rated oils. JASO is a relatively new rating specifically for engines that run the transmission and clutch in the same oil as the engine. You should be using JASO MA or MA2 (API rated oil of SG or lower) in your air cooled engines.
I switched from Honda GN1 10W40 to Silkolene 15W50 Pro-4 in my 97 air cooled 250 when I found I was pushing a 300* oil temp. I never had a clutch or motor problem with that bike. I sold it in 2004 (when I got a liquid cooled bike) to a guy who still has it today. He has never had any engine problems either. I continued to use Silkolene 15W50 in my liquid cooled engine.
I changed the oil every 3rd race, the guy I sold it to doesn't ride it as hard as I did and he doesn't change the oil as often, but I don't think you can change the oil too frequently. In my opinion, the more often the oil is changed, the longer the engine will last!
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