its not likly to matter all that much providing you dont use it into the colder months...good lubrication depend upon easy flowing oil at all temperatures...the heavier the oil the harder it is for the oil pump to move it around...you may have see this toy at garages to advertise lucas oil products.. you turn the 2 cranks and you can see that the one with the lucas additive stays on the gears better, what isnt mentioned is that it takes a lot more effort to turn the crank with heavier lubricant, in real engine life this means more gas to power the same engine. if you look at different grades of the same brand of oil you will notice that 5-20,5-30,10-30 usually say somthing about being fuel effecient, this is never stated on 10-40 or 20-50