To an extent, but you will hit a point of diminishing returns. At cold start-up, where you really want immediate lubrication, the 5w-20 will flow about the same as the 5w-30 (that is the "5w" rating). As the engine warms, the 5w-30 will maintain a higher viscosity because in my opinion you haven't hit that "dimishing return" point yet. You won't see very many people running a 40 or 50 weight oil because the viscosity is so high that the cool engine flow characteristics suffer. It is my understanding that something like a 5w-50 is not practical because the added improvers neccesary to achieve this viscosity spread will shorten the life of the oil a good deal. Remember also that flow and pressure affect each other. If you had a manual pressure gauge on a car with 5w-20, it would have noticably lower oil pressure than the 5w-30. I don't really want lower oil pressure at 6000rpms.
I am not an expert or anything, this is just what I have read.
The one I can't figure out is why Cam manufacturers are so against using a synthetic during break-in when you want to minimize friction......I don't think its the same principle as breaking in rings is it? Maybe I am wrong, and you need a certain amount of friction....
