14W50
I have 2 Gipsy powered vintage machines. One runs on 15w50 and the other on 100 straight.
Consumption on the former is IRO 1 litre every 2 hours and the latter twice that.
The only real way to test it of course is to operate the same engine on both oils and compare. However, suffice it to say I am very happy with the multigrade despite it being more expensive.
Buying oil by the box ( 12 (?) x 1 litre) is a lot cheaper. Buy it by the 20 litre barrel and invest in a trolley and pump - huge saving.
My R44 ( Lyco 540 injected) is running in on straight 100 and glugs a litre every 2 hours....
HP