Done my oil changes the same way on all my vehicles and I’ve never had any engine trouble. The longer the better theoretically. But after ten minutes, the advantage gets smaller and smaller faster and faster. Let it drip until you count six seconds between drips, or ten drips a minute, and you have given your best.

Engine Oil

The trick on how to drain transmission fluid is to work from the top, sucking out the old fluid up through the filler tube. Then refill with fresh fluid. A hand-operated vacuum transmission fluid pump makes the job simple and clean. You can remove one-third to one-half of the fluid from the transmission at a time.

I normally wait at least a minute maybe two. If you change regularly, the residual left inside is negligible. Especially if you changed it while the engine is fully warmed. All detergent oil, synthetic or not, gets dark as it retains more contaminants.

Once you’ve cracked the drain plug loose, position the drain pan under the oil drain plug before you completely remove the plug. Once all the oil is drained out, it will reduce to a slow drip. Don’t wait for the drip to stop because that could take days – a slow drip is fine.

Probably unnecessary to leave it out overnight but shouldn’t hurt anything. I don’t suppose a foreign object would enter the pan and contaminate it while open to atmosphere. A small quantity of dirty engine oil will probably still remain trapped in the engine even after an overnight drain.