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For the umpteenth time in the last fifty-odd years, I'm about to change the engine oil and have always followed the practice of doing so on a hot engine since the oil is supposed to drain more freely when hot. But hang on a moment -- since the whole point of modern multigrade oils is that viscosity *increases* with temperature, would it not be better to drain it when it's cold? In addition, if the engine has not been run for a few hours -- or days -- then the oil will have drained off the up-and-down and round-and-round bits back into the sump. And when the oil runs over your fingers as you remove the sump plug, you won't get scalded either. Yet I see that warming up the oil is still recommended in certain quarters. Is it really just a matter of viscosity, or have I missed something?
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Multigrade viscosity doesn't increase with temperature - it just doesn't get as thin as a non-multigrade would.
It's a good point about all the debris being more likely to be already in the sump if the engine has been standing for a day. I suppose the point is it would tend to stay there rather than creep sluggishly along to the drain hole.
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I guess the perfect solution would be to light a small fire under the sump to warm the oil up?
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You could get a fan heater and direct it at the sump for 30 mins to warm it up?
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The oil drains more quickly if it is hot. I doubt if running the car to warm it will make much difference to all the swarf and nuts and bolts swilling round in the sump. The strainer on the oil pump pickup is supposed to keep them there.
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I always drain my engine oil cold, without it having been started that day. All the oil and debris will have drained into the sump overnight, when it was warm, and then drains out into my oil container - it needs leaving an hour or two to get all the oil out.
Having refilled it and changed the filter, which being horizontal loses all it's oil, I start the engine and let it idle until the oil pressure light goes out. Let it stand again and top-up to the correct level.
Oil consumption is only 0.2 litres / 6,000 miles at 95,000 miles so any extra wear from idling empty until the pressure builds up is negligible.
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I doubt if running the car to warm it will make much difference to all the swarf and nuts and bolts swilling round in the sump. >>
I agree m'Lud, but what I had in mine was simply the old oil that would still be up in the rest of an engine has just been run.
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I meant of course 'in mind' not 'in mine'.
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I once did an oil change on the son in laws Carlton. Must have been a long long time since it was done. Drained down OK when hot into my special draining can but when I took it down to the local disposal point next day it was a different matter.
It was like draining out tar... and must have taken all of 15 minutes for it to empty.
So old oil thickens up when cold in my experience despite its origonal viscosity.
wemyss.
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I suppose the best answer would be if the manufacturers actually made the Sump-pan slightly "Conical" with the plug right at the point, then hot or cold, everything would come out very nicely indeed, inc all the "nuts n bolts".
Billy
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I think the ideal is to get it good and hot (on a journey) and then leave it long enough on your return to drain into the sump, but not so long that it is cold and viscous. That way it is thoroughly mixed, warm enough to drain out easily and not so hot that you burn yourself!
Small engines (e.g. on lawnmowers) without oil filters often have a magnet in the sump plug to catch swarf. This always seemed a sensible move to me, but doesn't seem to have caught on with cars...
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Of course, the premise of the OP is wrong, oil doesn't get thicker when it is hot, that's not what the numbers on multigrade mean.
Hot or cold - doesn't make a huge amount of difference IMO, except it takes longer cold - go and have a cup of tea while it's draining! Remember, there's always going to be an oil gallery full of oil that you don't empty out whatever temperaturew you choose to do the draining at.
If you change 99.9% or 99.99% of the old oil, it really isn't going to amount to a whole hill of beans of difference.
As long as you change the oil at reasonable intervals, as specified by the manufacturer, and use the type and grade in your car's manual, and fit a decent filter, that's all there is to it.
On this forum, we have been saying for a while that the extended drain intervals are just a sop to the fleet buyers, and that it isn't good practice. Is there any evidence for this view witnessed by an increase in the number of engines going bang? I think not! If there were, the technical side of this forum would be full of unhappy people complaining, and asking how to measure crankpin ovality!
There's no need for a magnet on most cars, because there's a full flow filter - I haven't seen many lawnmowers with a filter of any kind, although grass cutting technology isn't really my thing! The obvious exception was the gearbox in sump BMC engines. Their sump plug magnets were always full of swarf at oil change time!
Number_Cruncher
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maybe the magnet in the sump was to help the karma of the gudgeon pins ;-)
personally i like to get the oil warm and then have a nice pot of tea-a and then drain the stump
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Excellent bb, that's more like it :o)
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"Is there any evidence for this view.."
There might be if fleet managers kept their cars longer, but if they did, the manufacturers might be more cautious! Neither party cares if the engine goes bang after 10 years, even though it could last twice as long with more frequent oil changes. I care, because I drive old cars, and it's a simple and cheap insurance policy.
"I haven't seen many lawnmowers with a filter of any kind"
That was my point - the magnet is a (rather basic) substitute, but I've seen magnetic bands for filters that seem sensible enough. If the BMC sump plug magnet worked (with a filter) why not in sumps without a gearbox?
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"Is there any evidence for this view.."
Well when my Laguna went bang one of the mechanics put it down to the 18k service intervals !. Shell back in the late 90's had a whole field testing unit working with MB in Germany looking at the relationship between oil quality and mileage. Running Taxi's in Duseldorf on various grades and them taking them apart and puting back together.
Cars tend to follow truck technology back in the late 80's it was 6K service intervals on Mineral now you've got Trucks on 90K intervals some like the volvo's on Varible doing in one case 118K thing is a truck is designed with reliablity in mind not cost. Note VW are going CR because PD is to expensive to make. Most makers are of the view the customer won't pay for it. Plus cars have a design life whch is all to short will we expect to see as many Volvo in 20 years time as 740's .....
Paul
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On this forum we have been saying for a while that the extended drain intervals are just a sop to the fleet buyers and that it isn't good practice.
I change my oil at 6 months or 5,000 miles regardless. My 2 litre auto Mégane is now 11 years old and has done some 85,000 miles. Every year we take it for a month round France. Last year the route was from Normandy (ferry) to near the German border, down the Vosges and Jura mountains, round Lake Geneva, back into France then the the high route -- la grande route des Alpes -- down the Alps to the Med, across Provence, the Ardeche and back home. 2,700 miles, of which 2,200, were in France. All the mountain stuff involved several climbs/descents daily of 3,000-4,000 and sometimes 6,000 feet,the high point being 10,000 feet.
Over the entire trip, I used 500ml of oil and attribute this to the short oil-change intervals.
While I'm on, I've noticed the occasional slagging-off of French cars. Well, in those 11 years/85,000 miles the Mégane has needed one wheel bearing, one steering joing and an ignition coil plus the usual service 'consumables' which now includes discs. It still has the original rear brake shoes and exhaust system. As well as the oil, I make a point of changing *all fluids* -- coolant, brake fluid, auto box fluid, power steering to the maker's schedule and am sure that the failure of some owners to religiously do this lies at the root of many troubles. I know of main agents who don't bother, and have sacked them for my present pernickety independent.
Next week we depart fro the Pyrenees.
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Tend to drain it hot, that's usually because I've just got back from the motor factor (or supermarket) with the new oil! Makes very little difference IMO, except the time taken for the oil to drain. Is also easier to clean the bowl it's drained into if it's not had the chance to cool down.
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