Recomended oil weights became much reduced in the mid 80's. This was because a liter oil would improve gas milage slightly and there is a tax penalty if the corperate gas milage of all cars sold by any one maker was below a certain number. I have to assume Volvo was less then ideal and so recomended a lower viscosity. That being said, a lower weight oil will flow quicker and better to oil all the engine parts sooner after engine start up. Start up is when most wear occurs as no oil is in bearings. Heavy oil will take a few seconds longer to travel up to the head and down and around the crank. But heavy oil is better once the engine has run for five seconds or so.

Bill D.