Engine Oil
 

In the Bristol 407 handbook, it advises using, as engine oil, Mobiloil Special 10/30 or Esso 20/30 or Energol SAE 30.

This is odd as most V8s of this period are recommended to take a 20/50.

What is the best oil to use? A single grade sae 30 or a 20/30, as apparently recommended by Bristol, or the widely used 20/50.

In a similar engine Jensen recommend a 20/50, so why are Bristol, for a near identical engine, recommending only a 20/30? Does anyone know?

Years ago when I had my 408 which has the same 313 cu in engine as the 407 I ran it on 20/50 oil both pre and post engine rebuild, it was an oil burner before the rebuild and afterwards used hardly any between changes.
I also used 20/50 in my 410 which is of the same series and very similar to the earlier engine and by the time that came out Bristol were recommending 20/50 in the handbook.
I do use a straight sae 30 in a early 50's R Type and comma classic 20/50 in the other with no ill effects, my 603 has the same oil in it and personally I would prefer to use a 20/50 oil in an early v8 Bristol rather than a 30.
Not sure why the recommendations change it may simply because by the mid 60's the use of 20/50 oil was becoming more common and it was seen as an improvement over the earlier specifications and more suitable for sustained high speed on motorways.

Thank you.