I think you actually need to use synthetic oil in speeder bikes, because unrefined sarlacc pit monster excrement (crude oil) is often prohibitively expensive.
Now, if you're running your speeder in Dagobah, Endor, or Naboo, generally kind of temperate climes, you're going to want variable weight, say 20w50 synthetic oil (with Bandomirian ionite additives, because any other kind is worthless snake oil that will make your clutch slip) but Tatooine, you need an oil cooler and 50-60w. Some people might argue with me, but I can tell you: they're wrong.
Older models' repulserlifts are designed to run lean, and overheat in Tatooine, hence the oil cooler. SoroSuub will argue with this, but they DID put oil coolers in their later models, which is why I FIRMLY believe they are necessary in the older models as well, since overheating will limit your max altitude from 30 some feet to only, like, 5.
