OP; now that the work on the new block is nearing completion there will be plenty of space at the back of the RQ office to run the boat jumble if there’s interest.
I use 15W40 semi synthetic, either Halford’s own or Shell or Mobil. As long as it is rated API - CD or newer it should be OK.
My Nanni 115 engine takes 13L of oil which is quite a reserve carrying a lot of protective wear additives and plenty of capacity to cope with to cope with acidic combustion products.
The engine’s original oil spec’ is API-CD but that’s hard to get now, and the lazy revving engine with a turbo has been happy with the higher, newer specification oil and a good run now and again.
It still gets changed every 12 months.
I hate changing the impeller, space is tight between the front of the pump and the hatch screws. The old impeller is always in good condition when it comes out, unlike my hands.