MrTwisty said:
I use Lucas synthetic. Much cheaper and I bet my final drive lasts just as long.
I use Castrol Syntrax, as above I can't see it making any odds to life, any decent synthetic should be more than up to the job, regular changes will be far more important than how much the oil costs.
Coming from a GS I have a fear of impending Final Drive Failure, on the BMW I changed the FD oil every tyre change (as you needed the wheel out to do it) which equated to about 4k, on the Yamaha as it can be done without removing the wheel I will do mine every 6k (every service) it is only about £3 for each fill so not worth scrimping on when you consider in the same mileage I burn over £200 of tyres, £900 of Fuel and £50+ for engine oil and filter.