Does a stronger engine need a higher viscosity fluid? What's the reasoning behind this?
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Viscosity is not actually the only consideration because viscosity is a loss mechanism. What's required is for the oil to lubricate. If you could have a fluid that would keep the surfaces of a bearing apart with no loss (viscosity) at all then that would be good news.
The reason for needing viscosity is that we need to get the lubricant to the surfaces as they dip into the lubricant or it's pumped between them and to stay there. The oil in the main bearings acts as a cushion to protect the bearing surface each time the piston is driven downwards by the expansion in the cylinder and it has to be kept within the bearing and not allowed to 'squish out' each time. So viscosity is needed for that.
But it's not so much the power of the engine that tells you what viscosity is needed; it's factors like the loading of the bearings. A massive engine with big, low stressed bearings may be much less of a problem than a tiny motorcycle engine with a high compression ratio, high rev speed and small bearings.
The viscosity must be appropriate at all running temperatures, too. So a freezing cold engine can be damaged if the oil is too thick to circulate yet a thin enough oil to get over that problem may be too runny when the engine has been running for hours in mid summer. Hence, multigrade oils were invented.
Bottom line is that lubrication (tribology) is a very complicated business with many factors to be considered. How deep do you want to go into this?
Generally higher viscosity, which results in a thicker film of oil on surfaces at the same running temperature, or the same thicknes of film of oil if running at higher temperature, is needed when the clearances between parts is higher, or if the engine runs hotter. This would apply to all engines with these issues, not just race engines. I don't think modern race engines have the "slop" that was present in some old race engines, but they may run hotter than a street engine.
Another issue is that oil formulations have changed to increase the life of the oil and reduce the degradation of viscosity with time.
Standard car engines used to use 20W-50 oil changed every 3,000 miles, and if the car had an oil pressure gauge you could see the oil pressure falling over the 3,000 miles as the oil degraded and the viscosity reduced. Now they use synthetic 5W-30 or 0W-30 oil (made in a factory, not pumped out of the ground!) with much less degradation, and changed every 18,000 miles not every 3,000.
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