The Marina came in lots of versions here, with A, B, N and O series engines. You are lucky it's got the BW35 - BL made BMC use the Triumph Toledo manual box to get syncro on 1st. It was a horrible, horrible thing, and a lot were converted to the BMC box...as used in the MGB, which in the real (rest of) world was also used in every BMC badge car including vans like the J4. BMC's were pretty good cars, BL totally destroyed the brands reputation.
Nothing wrong continuing to do what has been working for you. If you are really curious, try a good 10w40 and see if it cranks or starts easier, revs more freely, consumes oil, leaks oil, or any other noticeable difference.