John
As Grant says the Merlin uses W-100 or WD-100 (one straight and the other detergent).
Aircraft nomenclature is different to everybody else and aviation 100 is actually what we would call 50 weight. The two almost universal mineral oils used in aviation are WD-100 and WD-120 (either 50 or 60 grade normal description) The straight oil is used in new engines for a short period of run-in before going to the detergent oil.
I would hazard a guess that 20/50 would be a good modern selection.
Aviation equivalent synthetic oils are becoming more widespread now as well.
Here are the Kiwis at it.
https://www.youtube.com/watch?v=yaAoI50KZ9Y
Lang