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post #11 of (permalink) Old 20-04-2017, 19:49
Man in the Car
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Join Date: Feb 2005
Location: Near Lincoln
Car: 06 MG ZR+120
Posts: 8,420
Originally Posted by Incony View Post
.. personally.. since the car had sat for that time... before even trying to start the car.. i would have taken the opportunity to change the oil.. since its had that time to settle in the sump.. and all the possible stuff that could move from the top of the engine to the bottom.. would have done... and you could have started with much cleaner oil by not turning the engine over ...
I am afraid it doesn't actually work quite like that - you are correct that all the oil and contaminants will indeed settle into the sump, but the solid contaminants will have settled to the point of forming a thick sludge in the bottom of the sump, and draining the oil out cold would leave this layer of sludge exactly where it is (and in all probability, the cold and viscous oil won't drain from the sump fully either).

You then fill with clean oil and run the car, and the settled sludge starts to loosen and mix with the new oil, rapidly degrading its quality.

I would always advise draining oil when it is hot, preferably after taking the car for a good long run, mainly at highish engine revs if possible. This will get the sludge stirred up and mixed with the old oil, and it will then drain out when you remove the sump plug. If drained hot, there will be an insignificant amount oil left after draining for five minutes. Even so, it is likely that some sludge will remain undisturbed in the sump unless you refill with a sacrificial oil as a flush - specific diesel engine only grade which has a high level of detergents in it and run for a few hundred miles in a petrol engine will stir up and flush out most if not all of the solid deposits in an older engine. Some people have even run diesel fuel in the engine sump for a short run of a few miles with very good results, although I have never been brave enough to try it!
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