The manual for it will tell you to use only 5W50 full synthetic and I used Castrol Syntec in my '96 FE501 exclusively for the 8or 9 years I had it. The my manual for my '05 says to use 10W50 full synthetic, but I have tried Shell Rotella T 15W40 (diesel engine motor oil) and had no problems. I think a number of people on this site would also recomend this or Delo 400. As far as change intervals, I usually change mine every 5 hours or so and the filter every other time. I'm not sure about the '99 models (my '96 did not have a filter), but my '02 and my '05 use the same filter as the KTM RFS motors.
