Really, the safest option might be to contact Casa or find their website & see what oil ratio is recommended. Or RLC may be able to clarify.
Whatever may be suggested on this Forum, any kit manufacturer should be prepared be aware of what is best for their product. Ray & Ben Kemp said @ the launch of the RB 250 that 4 % of 'the best' oils would be mandatory for that engine. But the necessity for that was acknowledged as to being due to the Boyeson feeds meaning scant big end lubrication, say, compared to a piston ported engine, like the engine that you have.
I am surprised that it should be necessary for you to be using that amount of oil.