All of the design validation (dyno and vehicle based durability testing) is done with factory-specified oil weight...it's the only oil that's been proven. Your oil performs many functions....splash lubrication, cooling, hydrodynamic lubrication for some bearings like main bearings and con rods, and some vehicles use oil pressure to operate the variable valve timing. You seem to be saying that you want heavy oil for heavy loads but hydrodynamic lubrication supports parts on an oil film...they never touch. The bearing clearances are chosen partly based on oil weight. As one Mazda engine engineer said, the oil is part of a system (actually part of several systems). In principle heavier oil creates more friction which creates more load on the engine and more heat, it takes longer to circulate on start-up, it can't be better for splash lubrication of cylinder walls, it's going to be less effective for cooling. Maybe you'll get lucky by second guessing the manufacturer or maybe you'll suffer unintended consequences of changing one component in your lubrication and cooling systems.
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