opti oil is pricey about a score for 4 litres but you don't use that much of it, it's at least a 100:1 mix. Th Optimax lumps are quite frugal , you might find you are only pulling 50 litres an hour at 5.5k revs.
Talk to the Rib Service Centre as they have an 8 mtr rib with a 225 opti and they know how much she consumes. Adam uses it quite regularly. They are in Southampton