I get this question all the time and since I have offered up my opinion at multiple points in the past I thought I would have an engineering collegue reply and here is what he said:
What do you think using the wrong weight of oil does to an engine? Using a 5W20 vs a 5W30… etc.
Well, from a general standpoint, I do believe that differences in engine oil does make a difference.
I had a vehicle for a number of years where if I used 10w-30 there was increased valve noise in the winter time. When I used 5w-30, the noise went away. This noise was repeatable (two winters in a row) so I am confident that the difference was the oil. I also know that it didn’t matter whether the oil was new or used, the 10w- oil had higher valve noise when it was cold out.
I know that noise is bad when it comes to reciprocating components like the valvetrain in your engine. Over time, flat spots will develop in the cam lobes and the rocker arm gaps won’t tighten up when the engine is hot. This will lead to a change in valve lift/duration affecting fuel economy and performance.
The valve train is only a small part of the overall damage that occurs over time. As an engineer that works with thermal systems & fluids, I know that the viscosity of lubricating oil is very responsive to the temperature. The kinematic viscosity of 10W oil at 0 degrees F is more than twice that of 5W at the same temperature. This affect is worse at lower temperatures. With an increase in viscosity in the winter, more of the oil is bypassing the filter, allowing debris to contaminate the oil system.
The same can be said for using light weight oil in the summer.
I’ve had a HD air cooled v-twin for a couple of years now and though it’s a good design overall, it does have a bit of a tendency to cook the oil. You can tell when you’re changing the oil that it is darker and when you rub it between your fingers, it feels more like water than oil (lack of lubricity).
Though HD recommends a heavier weight oil (20W-50) to compensate for the higher operating temperature, I think that they are less forgiving and should be changed before the mfr’s recommendations.
As a result of these experiences, I’ve been a little more cogniscent of the weight of oil that I put in my vehicles.
On the farm, we had 55 gallon drums of oil so as a kid with no money, what ever was in the barrel, whether it be straight 30 weight or low ash Rotella T for the diesels, it was good enough for me.
Pretty good answer I thought!