Originally Posted by John V6
Here is what I and a few folk on here in the UK use. I ran on my S1 and now on my 3.7l roadster

Morris Lubricants is the little company that the MMC has had a deal with for ages for their oils and lubricants. I imagine they copy the fluids/oil they provide from the component manufacturers specification sheet.but as they don't piublish specific specification sheets themselves, there is no way to confirm. But considering the lack of problems for 30 years, we can be confident. Sadly, they had a limited network overseas, nothing in Canada and only a guy from a basement (IIRC) in the USA. And, of course, such lubricants cannot posted be posted by air.

BTR precisely called for only one lubricant, Castrol SAF-XJ Sadly, Castrol substituted other fluids over time and SAF-XJ became impossible to find. However. the BMW 300 series used a similar BTR and one can pick up the right stuff, very expensively, from any BMW dealer in the world. I am adding the BTR part label below. However, I now see the BTR fluid back https://www.ecstuning.com/b-castrol-parts/castrol-saf-xj-75w140-500-ml/001989520310/

Synthetic 80w140 is no longer a tough fluid to find. The key is the addition of a friction modifier BTR began to use in 1999, after experiencing noises from their axle. It is a bit more technical than that, but one can read more.To end the noise, they used a "friction modifier", I believe from an Italian company called Sturaco. It is sold under many names, even by GM and Ford. So one can buy a high quality 80w140 and add a bottle. But these fluids last so long that it is unlikely a change will ever be necessary. BTR/BMW recommend 50,000 miles (80,0000kms). I change mine at half that. The used fluid always looks hale and hearty.

As long as the BTR specification is followed, the choice is merely a matter of cost price and easy sourcing. You won't get BTR to agree officially but over a pint they will agree.

gmg

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