There are plenty of charts giving the UK to NATO to North American specifications, but it won't be as simple as that. Oil specifications constantly change. You may have to look at what era your vehicle last ran, and on what oil was likely in there at that time.
Considerations include the detergent/non detergent criteria, and that oils are now different for diesel or gasoline. The removal of the zinc is a big one for flat tappet engines. With a lot of these older engines, either running diesel oil or else adding a ZDDP additive is likely necessary.
From my own experience, we changed from the straight weights to the multi weights pre 1980ish, and then went to the 15W40 on most of the fleets in the mid 80s, which we are still running for the most part.
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