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MG MGB Technical - Why 20w-50 oil?

I'm just curious. I've been driving since the 60's and in all that time the B is the only car I've seen that requires 20-50 oil. Why is that? In the past most cars took 20w or 10w-30 oil sometimes thinner in the winter. The only cars that used 40w were normally ones the owner had an oil burning problem with and he wanted thicker to last longer. I have no experiance with VW's so I do not know them. Whats wrong with 10-30 or 10-40 in our cars? Bob
Bob Ekstrand

The driver's handbook recommends 20w/40 & 20w/50 for operation in temps from -10 degrees C to 40+ degrees C

10w/40 & 10w/50 from -20 degrees C to 40+ degrees C

10w/30 from -20 degrees C to 10 degrees C

5w/30 & 5w/20 from below -30 degrees C to -10 degrees C

Chris at Octarine Services

I am no expert, but I believe that our engines to standard specification have larger bearing etc. clearances than modern engines. In moderate to hot air temperatures, 10W-30 would be too low of viscosity (thin) especially when warm making a lower oil pressure. I think the heavier 20W-50 acts like a bit of a cushion in the bearings also. I think it is all to do with materials and expansion as and when the engines heat up. Ours would probably seize under hot conditions if we had less clearance. Some of the newer cars use 5W-20, which also saves fuel, since it takes power to pump that heavier oil around. On earlier 3 main, MGA and earlier 20W-50 leaks out slower also! I believe some of the Harleys use 20W-50 also, so it must be a real man's oil! LOL (Just a joke, no offense to women MG or Harley owners)


Hi Folks:

I bought my first new MGB(66) in 1966 and the MG/Austin dealership serviced it every few months (3,000 miles max) with 30W non-detergent oil during the warmer months and 20W during the cold winter months on long Island. That was how it was serviced for 200,000 miles when its life ended. The 18GB original engine has been bored to 0.010 with nos pistons, and the crank journals are still within design clearances, and only requires a speedi-seal because of a groove in the hub.

I plan to use this engine when I re-engine my 67B, but I will probadly use a zinc additive because of the new oil formulations.

BTW the gearbox lasted just as long using 30W ND oil. I have rebuilt the gearbox with all nos parts and converted it to a "D" overdrive. I will continue to use 30W ND oil as long as I can find it.

Good luck: Rich Boris
Rich Boris

FWIW the BL Workshop Manual only quotes three categories:

Consistently above -10C/15F 20W/50, 10W/50 or 10W/40
Between 10C and -20C/50F and -5F 10W/50, 10W/40 or 10W/30
Consistently below -10C/15F 5W/30, 5W/20

You don't *have* to use 20W/50, it's not commonly available in the UK anymore being supplanted by 15W/40 in the case of GTX, and I've not noticed any difference on the gauge of the roadster, but did seem to get lower hot idle pressures on the V8. However I've tried Halfords Classic 20W/50 (which I wouldn't buy again as it is an unsealed can) and Valvoline VR1 20W/50 and didn't see any improvement.

What's far more important is the API rating of the oil, modern SL and SM grades don't have enough ZDDP and are known to cause increased wear on classic or 'flat tappet' engines. Stick to SJ or earlier. Diesel grades tend to have more SDDP (to protect the little-ends) and SJ is available in that when it isn't in petrol grades.
Paul Hunt

The B engine was made before the computer controlled age. The new machinery produces closer tolerances than were previously possible. This allows for the use of much thinner oils. Due to emission laws ZDDP levels are reduced every year in motor oils, even for diesels. I use Castrol 20w50 4T offroad motorcycle oil which still has the ZDDP package for reduced wear. I've noticed that that the oil pressure runs 10 lbs. higher than when using other oils. RAY

The oil I have been using in my V8 doesn't have the SJ type classification. Instead it says it is ACEA A2/B2. I can find that this now an obsolete specification, but does anyone know for sure that this is suitable for a flat tappet engine?
Mike Howlett


It should be according to:
Jean Guy Catford

ACEA is the European version of the specs, API is the American and is much easier to deal with, compare to

What oil are you using, Mike? All the ones I have looked at (but then I haven't looked at them all) contain both classifications. According to A2/B2 has been removed as it can damage *modern* engines. Looking at various sources A1/B1, A2/B2 *and* A3/B3 are all quoted as being equivalent to SL, which sources elsewhere say is not good for classic engines. You don't seem to be able to equate API and ACEA directly, API relate to years whereas ACEA seems to relate more to engine design, A3/B3 being quoted as for "High Performance - Turbo + Extended Drain" which hardly includes our engines! A1/B1 is described as "Fuel Economy" which is also hardly relevant. I think this is because ACEA only came into being in 1996, where as API has been going much longer.

Diesel oils *are* being pushed towards lower ZDDP content, but they still have more than the equivalent petrol by age.
Paul Hunt

This thread was discussed between 15/08/2009 and 17/08/2009

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