oils change massivly, when you start working an engine you'll tell the diffearance
with cheep crap oil the temps seem to run higher and when it gets hot oil pressure just disapears and it also ages a lot faster. on the other hand a top end premium oil will run cooler doing the same work, oil pressure will stay till it's a hell of a lot hotter and it will stay in grade a lot lot longer.
the mid range random brands are the ones to watch semi synthetic means pritty much anything you want it to