well the bloke that delivers the fuel here reckons if you keep 98 in 40 4 drums it should keep its grade for atleast 12 months dont know what others opinion are.
thanks for the reply guys being playing with the prop seem to be getting a bit better fuel burn it was too coarse full throttle straight level was only full 5200 revs. now got it set to do 5500 flat level. 90kts burning about 19lts