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Some oils are too thin and don't offer enough protection from friction. others are too thick and can cause other excessive pressure or not provide the lubrication the motor requires.
Not quite, Mike. You're assuming that the viscosity is the sole provider of keeping parts from rubbing. Back in the1950's that would have been true but advances in basestocks now provide boundary lubrication due to film strengths. Some have film strengths so strong that a modifier, like an alkylated naphthalene, is used to break it down so the additive package can get in to do its job. You'll be seeing a new oil coming soon in viscosity 0w-16. It is backward compatible with any of the engines. There are old style 302 engines on test stands today running 0w-10 oils and doing just fine. The biggest draw back is like you mentioned, you don't know what the internals are like if buying a used vehicle. Some of the high mileage engines can benefit and yield more miles by going to a heavier oil. Most of the High Mileage oils are on the thick end of the Kinematic Scale. I've recommended several times that you can add life to any of the Xw-20 speced engines by going to a Xw-30 oil at about 150,000 miles. The issue is the wear at the lower bearings. When worn they leak oil that would otherwise be sprayed on the cylinder walls for the ring packs. You can have good oil pressure and run the engine in an oil starvation scenario at the rings. The Xw-30 oils cures the issue by providing more resistance to flow which increases the oil pressure at the bearings.
The oil pumps in all engines have a pressure relief valve on them. There is little to no danger of too much oil pressure. Oil viscosity is mostly irrelevant as the valve body has no clue what the viscosity is. All it knows is when the spring in the valve body is pushed hard enough to open the valve it dumps oil back in the pan. Most any of the engines can easily stand up to a 50 motor oil. You'll find the performance, current production Ford engines specing a Xw-50 from the factory.
Car Manufacturers spend a lot of money in research to determine the proper oil required for each engine
True but only within the requirements set by ILSAC. Any car maker that sells vehicles in the USA belongs to ILSAC. They are the ones that determine what oils will be used and the limits set on any additive packages. ILSAC is the one responsible for the reduction of ZDDP in our oils, not the Feds, not the individual car maker, ILSAC. Then it is the car makers responsibility to design an engine around the lubrication system and oil requirements.
