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Some time ago on one of the many posts on which grade oil to use (The Mini Forum), someone wondered how the viscosity of different grades compared at normal running temperature ~90°C. I've got the instruments so I thought I'd measure it. Compared Castrol GTX 20w50, Valvoline XLD Classic 20w50 and Castrol Magnatec 10w40. Interesting is that there is hardly any difference once at 90°C. I used a high end R&D rheometer to measure these so the results are very accurate. A difference of 5 to 6 mPas is tiny.
I suspect people's observations on better oils is down to the additives used rather than the running temperature viscosity.

NZmember wrote:Some time ago on one of the many posts on which grade oil to use (The Mini Forum), someone wondered how the viscosity of different grades compared at normal running temperature ~90°C. I've got the instruments so I thought I'd measure it. Compared Castrol GTX 20w50, Valvoline XLD Classic 20w50 and Castrol Magnatec 10w40.

Oil #1.pdf

Oil #2.pdf

Interesting is that there is hardly any difference once at 90°C. I used a high end R&D rheometer to measure these so the results are very accurate. A difference of 5 to 6 mPas is tiny.
I suspect people's observations on better oils is down to the additives used rather than the running temperature viscosity.

...interesting,does that not suggest that using a top quality 10/40 oil is by far the best bet ?.. better running from cold thru to normal temperatures..a lot of old cooper engines are multi rebuilds and i would have thought most engine wizards now would rec a top qual 10/40 ?.. my trusted engine builder for mighty and super mighty minis certainly does and refers to 20/50 as sludge !!!....my top choice is a top qual mobil 20/50 semi synth.. the engine seems to use less of this also compared to cheaper oils with the same viscosity..foxy52

foxy52 wrote: my trusted engine builder for mighty and super mighty minis certainly does and refers to 20/50 as sludge !!!....

when we did mighty minis we discovered that 10/40 would start the gearbox baulking after a couple of races, 20/50 would extend the life of the gearbox for the full season

10/40 might allow the ultimate release of bhp, but does a hard worked syncro box no favours whatso ever

10W60 is probably the best of both worlds....

Cheers, Ian

I'd like to say a big thanks to NZ member for going to the trouble of doing that and posting it up here.

I'd also suggest that Ambient Temperature should also be taken in to account when considering oil viscosity.

Viscosity is all well and good.

It says nothing about film strength, or additive pack.

I suppose the only true test is how quick stuff wears out.

In the shed wrote:Viscosity is all well and good.

It says nothing about film strength, or additive pack.

I suppose the only true test is how quick stuff wears out.

precisely..i have used cheaper 10/40 oils but nothing comes close to mobil...over the years i have found the film strength to hold up well,i also note less usage and discolouration from new...foxy52

Oil is an interesting science.

I had an old suzuki 410 jeep and ran it on 20w50 crap. I threw a load of molyslip in it, just in case it smoked itself too dry and it raised the rev limit by 1000rpm!

Castor in the mini, BUT with evacusump gear. Got to pull the moisture out of it.

It's all about the zinc.

jay weinstein wrote:It's all about the zinc.

And the best answer is Comma Sonic 20/50, at 1500ppm, as opposed to Millers zddp content which was enough. So obviously not. Castrol 800ppm, Duckhams 800ppm can't remember the rest but nothing was anywhere near the zddp that Comma Sonic has............