It really depends on the operating conditions, and it's best to go with the manufacturer's specified weight for your particular climate. Straying away from those recommendations is like playing Russian Roulette. If you use an oil which is heavier than recommended you end up having high oil pressure close to the oil pump, but you risk ending up with an insufficient flow rate at the journals which are smallest / farthest away from the oil pump. If you go lighter than stock, you end up with high flow rates but much lower oil pressure all around, particularly at the smallest / farthest journals. Just something to consider.
Edited by PsychoSane, December 24, 2014 - 01:58 PM.