Oils first job is lubrication. It's second job is cooling. Third is cleaning. If you choose an oil that is too thick, its flow rate through the bearing becomes too slow, and the viscosity drag is too high. Overall oil temperature will rise with the increased friction caused by the oil itself. The hydrodynamic cushion forms instantly in any pressurized plain bearing. If 30 weight oil forms a cushion that the engine operating parameters can not break down, then 30 weight is thick enough and switching to 40 or 50 results in NOTHING other than lost power and increased waste heat. An oil cooler lets you run thinner, not thicker, oil. 30 weight is "thicker" than 40 weight if it's running at 150 degrees and the 40 is running at 220 degrees....
Racing engines, which operate under bearing loads 3 or 4 times what a street engine will ever see, will have a much larger bearing clearance which can flow 50 weight oil at high volume. They get a new set of bearings every race....
[ January 09, 2005, 08:20 PM: Message edited by: Fuelrod ]