I ALWAYS bang a litre of cheap ATF into the engine oil 250 miles before I do an oil change.

ATF is full of detergents, keeps the inside of the engine lovely, and cleans out lifters etc.

You can also put a litre of ATF in a diesel tank now and again to clean the injectors... on a diesel when I do a fuel filter change I prime the filter with neat ATF.

My oil is still clean 1,000 miles after an oil change, not manky 10 miles after.