I've always flown the OS .40 FP and LA series engines on either a wood or MAS 10 X 6 prop and 15% fuel with at least 50% castor oil (50/50 castor/synthetic). This seems to be the best combination for me. Don't expect a lot of power from the LA .40, and if you want good performance look for a plane like the Phoenix Sonic 25. The .40 LA doesn't put out much more power than the .25 FX.

Hogflyer