engine oil

[CornishPete]

I wanted to change the engine oil in my bike this evening. I've always done it after the engine has warmed up but the battery is flat so I can't start it!
I did a google search to see if cold oil drains the same but now I'm more confused!

I always thought the lower the viscosity the thinner the fluid eg water=low viscosity=thinner, syrup=higher viscosity=thicker.

it seems multigrade oils actually get thicker (10w40 is 10 when cold 40 when hot) is this correct? as hot engine oil has always appeared thinner to me, or am I confusing thick and thin with high and low viscosity?
if anyone could explain in laymans terms it would be very much appreciated!

 

[J.F]

This is one of the most useful video's on engine oil

 

[J.F]

Answer is in the video from 6mins to 12mins

 

[CornishPete]

So it would appear that I've wrongly been thinking oil will drain more freely whilst hot when in fact its thinner and will drain more freely when cold?
if this is the case why do all the guides i've looked at start off saying to warm the engine up first? (just looked at Mobil website)

 

[Bill Edwards]

Warm the engine up if you want thinner oil, it does get thinner with heat. You have not been doing it wrong.

I think the confusion is that most oils give a cold and hot temperature (e.g 10/40) but not using the same kind of scale, it's hot vs cold. So 0/40 and 10/40 have the same temperature hot but the 0 will be thinner when cold. 10/30 and 10/40 will be the same cold but the 30 will be thinner when hot.
But in both cases despite the higher number both kinds will be thinner when hot than cold. Just a different measuring system designed to confuse you.


I've changed a horrific amount of engine oils and have done cold as well as hot (which is my choice by far). The difference is in the oil running out when cold to flying out a couple of feet from a side mounted sump plug well past the bucket you've put underneath! Big difference.

 

[dobbslc]

Yeah first time I changed the oil in my Toyota Surf hot it came out so fast it wouldn't drain into the oil container but just pi$$ed everywhere! Too hot to get the sump plug back in .....
Did it again last night cold with a MUCH bigger tub under the car.

 

[CornishPete]

Cheers for the replies, I knew the first number followed by W was the cold viscosity and the second number was the viscosity when hot but didn't realise it was two different scales, this explains why I couldn't get my head around oil apparently being thinner when cold.

 

[normspanners]

Oil allways gets thinner when warm I have change thousands of engines and always try and warm it thoroughly before draining, it is the long chain polymers that increase I believe as it warms up, imagine a worm lengthening as it gets warmer and so doesn't seperate as easily keeping the 'gaps' between the worms less. The gaps sort of allow the metals to touch together and then wear.
Also doing tests on oil after 6000Mls a 10w/ 40 went to a straight 80, in cold weather so as it ages the oil thickens and it doesnt resemble what was put in anyway, as it has that much carbon in it.
a cold engine on a winters day can take easily ten minutes to drain and you wont get it all out! whereas a warm engine will take 1-2 mins all done.

 

[pedrobedro]

I've always warmed the engine to drain oil and the evidence before your eyes should tell you that it is thinner hot than cold. Old oil when up to temperature is like the viscosity of paraffin opposed to treacle when cold. I use a 25ltr container with the side cut out as a catch tray, you don't miss much with that.

 

[rikrobson]

If you imagine the heat viscosity as a descending line on a graph then each rating has its own line. A mono grade is one of the single lines. The weakest lines are lower than the non w lines. So for example a 10w40 will have the 10w line lower then the 40 line
But the start of the 10w is higher than the end of the 40 so you get less of a gradient meaning the oil a stays closer to constant viscosity.
I hope that makes sense as it's easier drawing it than writing it

 

[langy]

So it would appear that I've wrongly been thinking oil will drain more freely whilst hot when in fact its thinner and will drain more freely when cold?
if this is the case why do all the guides i've looked at start off saying to warm the engine up first? (just looked at Mobil website)

Nope hot oil will always come out quicker and easier

 

[Spark plug]

Modern oils are designed to perform over the widest temp range as possible so you don't have to put winter oil in your engine. But hot oil will always run more freely than cold oil, so although the isn't a large change in viscosity the hot oil will slide over surfaces much more freely than cold oil.

 

[slim_boy_fat]

I use a vacuum extractor for oil changes, dead easy and no lying on the ground with hot oil running up your arm.

 

[dazza]

I use a vacuum extractor for oil changes, dead easy and no lying on the ground with hot oil running up your arm.

I have one but it leaves a lot of oil in the engine, maybe every other one it may be worth dropping it from the sump plug.

 

[Bill Edwards]

I won't use them for an oil change as I view it as the lazy method used by garages that aren't that bothered about doing a good job. Always best to remove the sump plug and get it all out when hot, though yes it's not always fun.
Sucking out what you can first might be a good idea, less to shoot across the workshop when the plug is removed!

 

[sardine]

Can't beat a big old Lister with side doors, drain it, chuck in some parrafin, get your arm in, and wash all the sludge out.

 

[slim_boy_fat]

I have one but it leaves a lot of oil in the engine, maybe every other one it may be worth dropping it from the sump plug.

I won't use them for an oil change as I view it as the lazy method used by garages that aren't that bothered about doing a good job. Always best to remove the sump plug and get it all out when hot, though yes it's not always fun.
Sucking out what you can first might be a good idea, less to shoot across the workshop when the plug is removed!

When I got it first [to service tractors etc] I tested it on my then car. After it had finished 'sucking' I dropped the sump plug and about an egg-cupful of oil came out.

Given that I have to do my servicing at the roadside [sadly, no garage/workshop ] and carry out oil/filter changes at least double the recommended rate mileage-wise, it'll do for me.

 

[The_Yellow_Ardvark]

Those vacuum pumps were made popular when in the early 1990@s when it was supper easy to strip out sump plug threads.
Not a nice job.
If used correctly those pumps will get most of the oil out. The common mistakes people make is not wait long enough to suck the oil out or force the wrong side pipe down the dip stick tube.

As all ready stated modern oil flows just as good warm or cold.

When I use a pump, I pumped up, wait and crack on with other jobs on the service.

 

[Crater Creator]

The viscosity of a multi-grade oil still varies with temperature, but the change is less than for an old-fashioned single grade oil.
Just about every oil gets thinner when hotter.
The two figures stated on multigrade just shows that at higher temperatures, the oil behaves as if it was a heavier single-grade. It does not mean it is thicker/more viscous when hot, like some people think.

 

[mike os]

Gave up removing sure, l plugs 3 years ago... having vacuumed and removed plugs to check and getting less than a cup out....