at what condition to change your oil?

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Sep 18, 2002
san jose
my dad have always told me to change my oil when the engine have reached operating temperature. his reason is that oil flow better when its hot, therefore, more oil will drain out when changing the oil, so i usually change my oil right when i just got back from somewhere, and when the engine is hot. however, i'm thinking that since i'm using multi-grade oil(5w-30). oil is thinner when it's cold and thicker when it's hot, so changing it when the car been sitting for a few hours or over night will get more oil out. plus, the oil is already seattle down into the oil pans. what do you guys think? do you change your oil when the car is hot or when the car has been sitting for awhile and completely cooled down?
Neither; I change the oil when the engine is warm. After driving, I let it set for, say, an hour before changing it. A hot oil filter and hot oil is difficult to deal with, I find, and when it's cold (overnight) I fear I'm not getting all the oil out. Also, I think it's hard on a cold engine with just-changed oil to start it.
The old reason for doing this deals with cold climates and for the fact cold oil flows less quickly. By warming the oil it will flow out of the pan quicker.
A multigrade oil simply means it's going to flow better in colder temps than a straight grade would. So no matter what the viscosity rating is on an oil, it will always be thinner when it's hot. So you should always change your oil when it's as hot as it can get, as it will carry more contaminents out with it. The first number in a multigrade oil (such as 5w) doesn't mean it's a 5 weight oil. It's simply an arbitrary number assigned to correlate to a specific cold cranking performance at a certain preset temp. So all oils designed at 5w30 or 5w20 or 5w40 must have a cold cranking performance within a certain window at -30 celcius. [ October 05, 2002, 07:14 PM: Message edited by: Patman ]
I lay out my tools, make my coffee drive of 15-20 minutes, and drain immediately after shutting off the engine. Rags & wrenches make it easy enough to avoid hot oil. As a child I learned draining hot kept the contaminants in suspension. Certainly, it flows better that way. David
I run mine until it is at normal operating temperture. At least 15 miles. I drain the oil immediately upon engine shut down. While the engine is draining I take an awl and drive a hole in the bottom of the filter and let it drain until it quits. I usually let the engine drain for an hour. By that time the filter is drained and cooled off and easy to remove. I prefill the filter and install. Then install the crankcase drain plug and torque to 18 ft. lbs. Fill the crankcase with fresh oil and leak check.
Anyone ever let it drain overnight? I like to get all of the old oil out as possible! [Big Grin]
There is a thread somewhere in which people obsessed with this actually drained the oil and then put a smaller vessel under it and let it drain all nite or longer to see how much more would come out. It wasn't much. IMHO if you worry that much about it you are not changing your oil often enough! Regards
I drain it hot. Drive till *well* warmed up, park in carport, position ramps, drive up on 'em, kill engine. Spread newspapers, position pan, remove drain plug. Let drain ~30 minutes or more. Hot oil removes more cooties! [Big Grin]
Originally posted by Chris: Anyone ever let it drain overnight? I like to get all of the old oil out as possible! [Big Grin]
I've been known to let mine drain as long as two hours, but never overnight. That's only because I usually do my oil changes first thing in the morning though. When I win the lottery I'm buying a house with a built in oil change pit dug into the garage, and then I'll definitely let the oil drain overnight! [Razz]
The risk of draining overnite is the dirt that can get blown in. If you have a dust-free environment, it wouldn't hurt. But always drain it as hot as possible. We use an air hose in the filler to blow out additional oil.
Does it make that much of a difference draining hot vs. cold? I usually change oil when it's cold (and give it an hour to drain), as it lets me reach the oil filter from up top without burning myself on the exhaust manifold. And I never cared for dealing with hot oil. But if it makes that much of a difference...
Draining it cold means you run the risk of not all of the contaminents coming out. With it being hot, they are in suspension better and will flow right out. If you don't want to burn yourself, do what I do and just let the oil drain a lot longer before doing the filter change.
I also drain overnight. If dirt can get in it first has to blow up into the drain valve and then compete with the oil dripping out so I am not really concerned about dust blowing up into a draining valve. I take the filter off the next morning.
If you drain it cold, you would have more of the oil in the pan from the top of the engine. This would give all the oil from the valve covers ect. and allow it to run off the internal parts. If you do it hot, you will not get all of the oil out, because it hasn't reached the pan yet. Then take a quart of oil and pour it into the engine (this could be warmed in the microwave) to flush out the remaining contaminates.
I knew a guy many years ago whose "purge" cycle was: Drain oil; overfill engine with diesel oil (has some lube properties); remove coil wire and turn engine over a few times to churn it all up; drain diesel; fill with motor oil and churn it up to get oil back in the bearings; replace coil wire and actually run engine until good and warm; drain oil and replace with the final fill of motor oil. He was also frequently known to remove rocker covers and drop pan to clean them out. RW
that's what i do with every used car I buy. I warm it up, drain a quarter, replace it with 1 qt kerosene, crank the engine, let idle for 5 min, and then drain it. Kerosene is the best cleaner, I think that's what the engine degreasers are.
Just drive the car until the engine is plenty warm, drive home, and drain the oil right away. The last few drips are insignificant; also insignificant is anything you might flush out by wasting a quart of new oil, but do it anyway if you wish. Do not use any home brew cleaners like kerosene, diesel fuel, ATF, or anything else. If you feel the need to clean your engine internals, use Auto-Rx The same product, half-bottle here, also cleans your automatic transmission. Ken
If you put kerosene in there, then when you drain it out, a little bit of it will remain, which will degrade (and thin out) your next batch of oil. I second the advice to use Auto-rx.
There's another reason to change oil hot or warm. The oil will still be present and warm in the bearings and lifters so there will be less possibility of knocking on startup. Even if you pre-fill the filter, as I do, the oil retained up in the passages will drain out. This is why you get a cascade of oil when you loosen the filter on a chevy engine even though the filter is upright and cannot drain down. The time it takes to fill the passages is danger time. Of course if you don't believe in backflow preventer valves in the filter then no worries, because it will do this every time it sits overnight.
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