I'm running the Bel-Ray Sport transmission in my '96. Baker transmission recommends this for all Sportster transmission/primaries. THe price is reasonable too. I think I paid $7 and some change for a quart.
30wt non-detergent motor oil works just fine. My Sporty has been running 30+ years with it in the primary. Even with the high cost of oil today, it won't set you back more than a dollar and a half a quart.