The old question of oil quantity

[Ignatzcatz]

Whilst I am fully able to carry out an oil change myself, a year had rolled round, the car was up for the MOT and I thought a continuing service stamp in the owners service book would be a good idea, so I booked the beast into Cridfords for the work. They poured in 10 litres of Mobil 1 0W40with a nice red classic filter. Driving the car back home I noticed the oil gauge needle was just on the upper red sector and the oil pressure was about 3.5 bar. I was a little worried about this quantity reading and the first thing I did at home was to dip the oil with the motor running. On the dipstick the level was midway between the two markers which according to my owners manual is acceptable but it does say the gauge needle should be in the white sector of the gauge. I feel that the car should have an additional pint or even a litre more of oil just to get that needle up a bit. Obviously I don't want to overfill the engine and start blowing seals but it does seem that it needs a splash more. Also I thought a 20/50 weight oil would have been a better choice for an old 3.2 although you cannot contest the quality of Mobil 1. I'm just a little concerned.

[Cortina]

I would "trust " the dipstick before the gauge. Qty seems grand , level seems right (better not high) , but I would have put VR1 in (yes - 20/50) , not what they have used (usual rules - ask 5 people about oil , it's likely you may get 5 different answers !)

[Phill]

Agree with above, trust dipstick level over gauge. My preference is to have the level about 3/4 up between the lines when warm. This then allows for expansion on very hot days or if you are pushing hard or stuck in traffic. Half way up is fine though.

[gridgway]

I have been advised not to fill to the top mark, so I do half way to three quarters.

But I don't think oil expands when it's hot though!

I assume the reason for measuring with the engine running when the oil is hot is the rate of return through the engine to the DS tank. With cold oil it takes longer to drain down to be scavenged back. You can see that on the guage when the engine is ticking over when cold as opposed to hot.

On the topic of oil weights, does anyone have an actual explanation of what makes a weight right for a particular engine? 0/40, 15/40, 20/50?

Graham

[fetuhoe]

The Coefficient of Volumetric Expansion for a typical engine oil is 0.00070 per degK.

I think that the reason for checking oil levels with the engine running is really concerned with the configuration of the dry sump tank and the oil pump levels.

Depending on the condition of the pump in terms of the clearance in the mesh, oil will drain from the tank and into the crankcase.

On some engines this can completely empty the oil tank within a few hours and if oil levels are checked at this time the system will be over-filled and all of the excess oil will blow out of the tank breather.

Later 996/997 GT3 Turbo engines have been fitted with check valves to prevent oil drain down as on these engines the Turbo would fill. These chack vales have been known to fail closed with resulting engine damage and I wouldhn't been keen to fit this tyoe of device to a Normally Asp[irated engine.

The correct 'weight' of an oil relatively complex and concerns the most suitable viscosity for correct operation.

Modern engine which run at very tight clearance use 'thin' oils such as a 0W/30 and if they are filled with a 20W/50 could suffer significant damage partiularly when cold.

Most Air Cooled 911s still have relatively 'wide' bearing clearances and also tend to run with resonably high oil tempertaures and work well with a 20W50 such as Valvoline VR1.

Using 0W/40 oils may not be an issue in terms of bearing operation but could result in an increase in oil leaks from the case if the engine stands for long periods of time.

[Strictly]

ive noticed, it typically takes 11.5ish - 13ish L for a full oil change from the sump and tank for those with the sill oil pipes, and how long it was allowed to drain. Of course every car is different.

As everybody else has written trust the dipstick over the gauge, and half way between min and max is just fine for a car level and idling with the oil up to temp, i would probably keep it at 3/4 myself.

Check your handbook for your oil pressure spec, it is normally something like 5 bar @ 5000 rpm @ 80 deg C (i just made that up...but the important bit is the triangulation, of the 3 conditions). So check all 3 at the same time, and if your in-spec then just top the oil up, if your lower than spec, change to a thicker oil....

Interesting comment from Chris above, on the Mobil 1

I personally would run a 20/50 or 15/40 depending on what oil pressure I was seeing from the car and how close it was to spec. I would run a 10/40 semi myself on a fully rebuilt engine once broken in on, but personally not sure i would ever use a 0/40 or 5/40 or 5/30 in a 3.2...but each to their own, there are merits, such as oil temperature degradation etc, depends how you drive etc. Who knows I might try it one day....

[Phill]

But I don't think oil expands when it's hot though!

Graham

It most definitely does!

[gridgway]

But I don't think oil expands when it's hot though!

Graham

It most definitely does!

But certainly not to the extent that Phill is suggesting, ie only fill to 3/4 to allow room for expansion. From 20 degrees to 100 degrees will give you 6% ish.

So your 3/4 of a tank turns into 4/5 of a tank! I'd also be pretty sure that there was contingency above the 'full' mark

Graham