Inspired by this question about how newer cars tend to specify lower oil weights, what would happen if you took one of those cars to an oil-change place, and instead of checking to see what weight of oil your car needs, they just dumped five quarts of the ubiquitous 10W-30 in?
Bonus question: would you be able to tell the wrong weight of oil was used before the consequences start showing themselves?
Quite likely, nothing…
A higher viscosity oil, at the low end, would move a little less freely through the engine when cold. That could result in somewhat higher wear as the engine warms up, but over the life of the engine it would be very hard to point a finger at that oil change and attribute any damage to it.
The lower viscosity, among other things, allows for easier starts at low temperatures and probably reduces fuel consumption a bit. It might also allow for tighter clearances in the engine and turbo (if there is one).
I don't know of any practical way of telling that the wrong oil was used after the fact – other than checking the receipt.
Up until a few years ago you likely wouldn't notice a difference. Many modern engines now use a variable valve train geometry design. Basically a low rpm the engine has a lower valve lift this increases torque. As the rpm increases the valve lift increases increasing horsepower. The valve train change is a function of oil pressure. Higher rpm equals more pressure, more pressure increases valve lift. Since higher viscosity oil can cause an increase in oil pressure the valve lift may be incorrect for the engine rpm. This could cause poor idle or low rpm performance.
