Another reason for multiple viscosity oils is that at cold temperatures, the starter motor may not be powerful enough to turn over the engine with the recommended high temperature SAE number. In addition, the higher the starter motor RPM, the quicker the bearing will develop an oil thickness layer that gets past the dreaded metal to metal contact stage.
The name of the game is to maintain a minimum oil film thickness to prevent metal to metal contact at all levels of bearing load and temperature. The four primary factors that determine film thickness are RPM, bearing load, bearing temperature and oil viscosity. I don't think that you can come up with the exact SAE number to use with empirical bearing design formulas but they will put you into the ballpark to be then followed by laboratory testing for the particular engine application.
"After a lot of investigation, I used Mobil 1 MX4T racing oil, as per Triumph. BUT, where the Triumph branded oil is 15-50, the Mobil branded oil is 10-40 (or 15-40), which seems to be just fine."
Just fine? How do you know? The answer is that you don't know. It depends on the ambient temperature and how you drive. Maybe it's okay for your particular application, maybe not. But perhaps at various times you're getting metal to metal contact. You just don't know, but if that is happening, you are accelerating the life of the bearing and will experience premature failure.
Sometimes you see a higher SAE number specified for higher ambient temperature ranges. That means that testing has shown that for that application, metal to metal contact can occur under some conditions, so the higher SAE number is justified. But if the season changes and the risk of exceeding that ambient temperature has ended, then the normally recommended oil should be used. You could be worse off with the higher SAE at low ambient temperatures.
Now suppose that the manufacturer recommends the same oil for all ambient temperatures and you decide to jump up a number in hot weather because you think that it protects better at higher temperatures? Well, the higher viscosity oil takes more energy to shear and if the bearing is not capable of dissipating the resulting extra heat, then the temperature will rise. But the SAE viscosity curves are not straight lines and it is conceivable that at that temperature, the film thickness of the higher SAE is less than that of the lesser SAE oil. So then you can end up with metal to metal contact, just the opposite of what you were trying to achieve. So the following statement is generally, but not always true:
"So a 15W50 oil provides more protection once the engine is warm than does a 10W40 oil."
It only assures more protection if it is the oil recommended by the manufacturer for that ambient temperature range.
This is a complex affair and I myself always use the exact SAE recommended by the manufacturer. They should know way more than I about their application. I'm not going to risk the price of a new motorcycle or major overhaul for what is in relative terms a few extra bucks. Why is it that throughout my lifetime I have encountered scads of folks that will spend whatever it takes to obtain the highest quality stuff, but not when it comes to something so important as motor oil? Go down to the Triumph dealer and buy the right stuff. Having special motor oil formulated and then trying to market it at elevated prices just might be a scam, but then again perhaps it might not. And why go through all that trouble when there are easier ways to scam with products that they produce themselves if they wanted to do so.
As for the lower number in a multi SAE I think that there is a bit more leeway. My wife's BMW Z3 specifies 15W40 full synthetic, but unless I buy BMW oil, I can only find 10W40. I think that I can get away with that. Think? Or am I violating the statements of the last paragraph?
Finally, the primary caveat for anything written here: I am a retired mechanical engineer but have very limited experience in journal bearing design. If there are any real experts on the board here that can correct anything I've stated in error or can update us on more recent knowledge, your comments are surely most welcome.
1966 BMW R60
1969 BMW R69US
2004 Triumph TBS
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