Why don't you add coolant instead of oil in your engine? Oil doesn't play a major role in cooling an already watercooled oil. It has to act against friction ( does cooling system do that??) If the oil is too light, it won't stay between metal surfaces and would lead to friction between them. If it is thick, it will stop friction but it will be harder to start an engine and it can clog up valves in cars with any sort of valve timing systems (VVTi and VTEC). The grade is used to buy the optimal oil for the area you live in. For example, if i live on a colder area, i would use lighter oil because it flows easier, but light oil can't take alot of load, so oil has 2 grades. The first grade is to tell the load it takes on cold start. Second grade is to tell how much load it can take and how thick it will get on warm engine.