interesting conversation about draining oil

[tuareg]

Since I woke up it's been pouring rain. Might be the first day this month I won't get to ride. So my other bike needs it's oil and filter changed, might as well do that today. Was talking to one of my good riding buddies this morning about what he was doing instead of riding today, and he was basically doing the same to his bike on this rainy day.

He says "are you going to warm up the bike before draining the oil?"

I said "I always do, don't you?"

He says "nope, the last time I rode it the oil was warm and when I parked it, all the warm oil drained down into the sump".

I said "yea?" questioningly, like, where are you going with this?

He says "well, if you start it again you are just going to push dirty oil from the sump up into the top of the motor again, where it has to drain back down, so you can drain it from the sump".

I said "That's true, but won't it drain faster from the sump when it's warm?"

He says "Ever open a bottle of oil that's been sitting in your garage for a few weeks?"

I said "yea?" again wondering where he's going with this

He says "did you have any trouble pouring it out of the bottle?"

Hmm now he's got me pondering. Since I was a little kid I was told to heat up oil before draining it, but he's got me thinking now. All the dirty oil did drain down when I last came in from riding and parked the bike I have to assume. Do I really want to push that dirty oil back up into the top of my engine now?

Oil might not get changed until tomorrow. I have to work this out in my head. He may have a point.

What do you think?

I had almost the same conversation with the guys who came to service my sailboat's diesel engine a few years back. I usually do that myself, but since it was in the high 40s(deg. C), typical Mediterranean summer, and I didn't want to bother about getting rid of the used oil, I called the Volvo Penta service. Anyway, 10 minutes after the they arrive they are already pouring fresh oil into the sump. Only then I realized that they hadn't even asked for the ignition key. Wadda heck? Their argument was the same: Do I really want to "contaminate" the uppers and the oil passages with dirty oil? Well they're the authorized service, what do I know? So off they go. First time ever I have seen a cold oil drain. Then I start thinking:
- Why am I draining the oil in the first place?
- to get rid of the dirty oil.
- only dirty oil?
- no, the detergents, viscosity index improvers, extreme pressure agents, anti-wear additives, dispersants, anti-foam agents, corrosion inhibitors and a number or other additives are at the end of their useful lives, either depleted are burned up or whatever. So I need to get rid of the old stuff as much as I can.

There's some basic science here; that dirty stuff had been circulating in those passages until I switched the engine off last time. So it's not going to get any dirtier because I decide to warm up the engine. All that stuff will settle down in the sump after switching off, just as it did the last time. The only difference between warm and cold oil drain is, warm oil drains faster(due to its lower viscosity) and leaves a relatively thinner film on the oil-wetted surfaces. So there will be a smaller amount of dirty oil remaining at the bottom of the sump and in its nooks and crannies(oil from the uppers having already drained back to the sump). The oil filter retains the most of the particulates; the only thing the oil filter does not retain is the heavier particles that have not been picked up through the oil pump intake strainer from the bottom(we usually see those after splitting the engine apart). So a warm oil drain will help a little in this respect too.
Viscosity of a typical multigrade oil(say 10W-40) at 60 deg. C is very roughly ten times less than it was at 10 deg. C(after that it tapers off slowly), so warming up the engine until the oil is in the 50-60-70 deg C region should do the trick, meaning getting rid of as much old oil as possible, with less chance to burn one's fingers.
Having said that, with those modern engines and synthetic/semi synthetic oils, the difference between a warm drain and cold drain could be too small to notice within the useful life of the engine(or of me) so both methods are viable. Then again, a warm drain provides the excuse for the extra trip to the supermarket in the next town which happens to be 100 km away, so I go for the warmer alternative. Just my 2p.