I have read on the engine oil bible website how bad it is for an engine to be overfilled with oil and that when the level on the dipstick is at maximum the oil level in the sump is just clear of the crankshaft and that any more oil will allow the crankshaft to whip the oil up into a foam or froth.

My question is :

if an engine were overfilled by say 250-500 ml, and the engine was used in this state for 200 miles before the excess was removed, would the appearance of the oil left in the engine after these miles give any clue as to whether or not the oil had been made foamy by the crankshaft and if the oil had lost its viscosity or been damaged ?

there is nearly as much rubbish spouted about oil as there is about tracking.

if the oil level when correct was just clear of the crankshaft the theory would be fine if we are talking stationary engines. vehicle engines are subject to forces from acceleration, cornering and especially braking. older engines used unbaffled sumps and when driven vigourously the oil spent more time getting to know the crankshaft, etc than it did waiting to be ****ed up into the oil pump; half a pint down could easily result in lost oil pressure. as far as i know this has never hurt any oil molecules.

to combat these problems modern engines tend to use heavily baffled sumps. it must be realised that the oil that is pumped around the engine also is beaten up by the thrashing internals as it falls under gravity back to the oil pan. if this process hurts the oil in any way then it wasn't very good in the first place.

keeping the oil level correct is important but for a different reason. if too much finds its way under the pistons it is possible for some to defeat the piston rings and enter the combustion chamber. heavily swilling oil might also force its way into the crankcase breather system and be drawn into the combustion chamber with a variety of effects not the least of which is the now frowned upon puffs of blue smoke. a small amount of oil being burned on a regular basis is likely to shorten the life of the catalytic converter. a large amount of oil is likely to cause severe engine damage.

a diesel engine can run on lubrication oil and a runaway engine is not a joke.

When I have oil change, I have the desire to fill the oil pan up to the maximul level indicated on the dipstick, and the result is always overfill, something like 200 to 300 ml. But I am not worried about this extra since, as a mechanical engineer, I am used to designing with a factor of safety from 2 to 2.5. But once I did overfill to 700 ml. After a one-hour trip, I checked the dipstick and I notice there was foam around--which means air molecules are physically mixed with oil. Going by the book, I concluded that the extra oil could result in less viscosity. I took my time to drain the extra 700.