Why warm your engine before an oil change???

[pothole]

Originally Posted by Meat Head

I understand the slow draining of the flat six. But with my method, the car has been draining for 12-15 hrs. (It has all gathered in the oil sump over night). As far as the sludge, your car is at operating temp when you park it, so I'm assuming the sludge will settle in the oil sump at this time.

I could be completely wrong, but I think the oil companies suggest bringing the car to operating temp because many people don't have the option to change the oil were the car sits overnight (without starting). Therefore, if you start the car (pumping the old oil through the system) it is crucial to bring it to operating temps to not leave the sludge behind and allowing the oil to flow.

I'm not an expert, just my theory lol.

Hmm. In my experience the rate of flow becomes absolutely teeny after about an hour. Can't really see the benefit of leaving it overnight. However you do it, there's going to be a fair bit of old oil left. Well, unless you fill it, run it and flush it a few times before refilling.

If you want to really reduce the amount of old oil residue, I think you need to completely fill it after draining, run it for a while and then dump and refill. Leaving it for hours and hours to drain doesn't make much difference to the residue stuck to all parts of the engine.

Personally, I just drain it for about an hour from warm and refill but I do my changes fairly frequently - around every 5k miles - so I really don't worry about a little residue. Fortunately, these cars take a **************** load of oil, so any residue gets pretty well diluted.