Myth- always change you oil on a warm engine

[Bob V]

I think we all know the saying always change you oil when the engine is hot. This dates way back to when engine oil was single grade and acted quite a bit like molasses, oil wouldn't even drain when cold. Today multi-viscosity oils are a lot thinner when cold.

Now, think about what is going on in your warm engine when you shut if off. Oil is everywhere from the top to bottom, it takes a while for all that oil to DRIP down, even when warm, and with 15W-40, I the oil is acting like 40 weight. It will still be dripping after several hours.

Now if the engine is cold all that oil that was up in the motor has dripped down and in the pan. The "cold" oil is now acting like a 5, 10, or 15 weight oil. How much longer do you think it will take to get all that oil out of the pan? You are probably putting the same type oil back in, do you heat it to get it to pour back in the engine, or does it flow out fine? I just cant see there being a real difference between a draining oil between a hot and cold engine. It will still take a while for everything that can come out, to come out. Also, I have found that the oil filter is not as full on a cold engine and it wont make much of a mess when removed

Now, this only applies to "normal" temperatures. If your ambient is below 0F you might need to warm it up a bit, but I look at it this way. If I don't have to heat up the oil to pour it in, why do I need to heat it to drain it?

Just my opinion, you are free to ignore if you want, but why take a chance on burning yourself if you don't have to.

 

[AMink]

I've wondered about this. I still warm it up and have a Fumoto valve so no burning necessary. I usually let it drain for about 45 minutes while I do other maintenance stuff on the truck. It makes sense that old oil will be at least as viscous as new oil and should pour easily. Maybe the hot oil "detaches" from internal parts better than cold oil.

 

[Wayne M.]

I think a hot drain still is the best to get impurities out, and besides, if you wait a while, the oil will usually siphon back out of the filter, similar to a cold drain. I guess it's just a matter of preference.

One interesting thing I have just discovered. While changing my own oil pan, I was startled when I saw all the oil still in there after a few hours long drain (I wanted to ensure a drip free engine). The reason is the bung in the pan is a lot thicker than the pan itself, unlike a passenger car or light duty pan where it's a thin plate spotted on the pan. Here's a picture... So we are never getting "all" of the oil out.

 

[JR]

How much do you think is trapped in there Wayne? A pint? Or less? It is hard to judge by the picture. Was your new pan the same way?

 

[MLee]

I am going to install a Fomoto plug when I change the oil. How far above the bottom of the pan does the threads stick up? trapping how much oil?

The pan has numerous slots in the threads, around the original drain plug for oil to get out when loosened.

The question is does one cut some slots in the top of the treads of the Fomoto plug to allow all the oil to get out?

 

[Bob V]

I think a hot drain still is the best to get impurities out, and besides, if you wait a while, the oil will usually siphon back out of the filter, similar to a cold drain. I guess it's just a matter of preference.

One interesting thing I have just discovered. While changing my own oil pan, I was startled when I saw all the oil still in there after a few hours long drain (I wanted to ensure a drip free engine). The reason is the bung in the pan is a lot thicker than the pan itself, unlike a passenger car or light duty pan where it's a thin plate spotted on the pan. Here's a picture... So we are never getting "all" of the oil out.

View attachment 86189

There are a lot of pans that way. I have seen cars and trucks that have two sumps in the pan because there are steering and axles clearance issues. Only the large sump will have a plug, leaving a quart or two in the other sump without any way to get it out.

 

[Wayne M.]

That is the new pan. Hard to judge but I don't think it was a pint. When I was sliding the old pan out I didn't look and picked it up by the front. Now there's a horrible black hole in my driveway. Lol. Thing is in your mind, you're doing something, but its not quite what's happening.

 

[AMink]

I am going to install a Fomoto plug when I change the oil. How far above the bottom of the pan does the threads stick up? trapping how much oil?
The pan has numerous slots in the threads, around the original drain plug for oil to get out when loosened.
The question is does one cut some slots in the top of the treads of the Fomoto plug to allow all the oil to get out?

I've seen threads where guys drill holes in the Fumoto to let the oil out. I change mine every 5,000 so I'm going on the premise that 11. 5 quarts of fresh oil will dilute 1/2 quart dirty to the point where it won't matter.

 

[DZink24]

my thought is that when you do it warm, the sediments in the oil haven't had time to settle in the bottom of the pan. shut the truck off, pull the plug, go do something else for a few hours, come back and put the plug back in, change the cool filter, and refill. . if you go out and pull the plug on a cold engine, all the "nastys" in the oil are settled in the bottom of the pan, and not in suspension.

 

[BIGNASTY]

I make sure its good and warm (norm op temp) and bring it into shop pull the plug and filters and will continue the next day. Have stuck my Pinky in the pan drain and can just get a finger tip wet, must be draining pretty well.

 

[Larry Willard]

I took my Fumoto out of the '01 and there was no more oil to drain, I would see no need to drill or cut holes in it.
Larry

 

[moparguy]

my thought is that when you do it warm, the sediments in the oil haven't had time to settle in the bottom of the pan. shut the truck off, pull the plug, go do something else for a few hours, come back and put the plug back in, change the cool filter, and refill. . if you go out and pull the plug on a cold engine, all the "nastys" in the oil are settled in the bottom of the pan, and not in suspension.

My thoughts exactly. My rule is always change while hot. I've got an old heavy duty rubber glove I put on in case a little hot oil want's to get too friendly.