Always use OEM weight oil as printed on the oil cap or in the owner's manual, unless you have totally rebuilt the engine which could change the bearing clearances. That's really what it boils down to, folks. The viscosity rating may need to change if an engine builder says so. But if none of that has changed in the motor, changing viscosity runs the risk of oil not flowing properly throughout the motor.
Remember this if nothing else: the oil pump has a volume or capacity it flows throughout its rpm range, and those values don't really change over the life of the engine. Once oil leaves the pump and encounters restriction (a main bearing, rod bearing, etc), oil pressure is realized. Sort of like sticking your thumb into the end of a garden hose. Do you really want to mess with this equation by changing the thickness (and therefore the flow rate) of the oil? The answer should be no. Oil is the lifeblood of the motor.
Me, personally, I always use synthetic but that's because of how I drive currently in Germany (vroooom) and because of how well they keep things clean inside the motor. Yes, synthetics were around in the 90s.