What ever clapper rings you're bell.. For me the fully synthetic oil can leave the engine like a Welshmans garden... Full of leaks... Simular thing with the semi-synthetic. The rule of thumb I use is pre-65, 40 grade and 20/50 for younger motors. Change it regularly, specially at this time of the year so the motor has nice clean oil in for the Winter. Depends on the mileage, usually 1.5k Another change before going to the Manx. Trials/long distance reliability Cub gets oil change every 1k miles and I have never wrecked an engine due to oil failure... Just make sure it is good oil and not this premium grade, costs a fortune and contains more additives than an old motor needs and is expensive. Castrol, Morris's, Elf and various grades are not too costly and do a great job... Griff