What oil

I have a 2005 1200 Sportster what oil should I use in the primary/trans..do they not run in the same?

Seems H-D is now selling a "one size fits all" synthetic oil (I don't buy from H-D dealers) & while they may say that's OK, I don't buy that sales pitch. Especially coming from those who threatened to void your warranty for even using synthetic a few years ago.

Good ol' "SportTrans" (I don't think it's sold anymore) was 75W90 gear lube (about the same viscosity as 20W50 motor oil) with additives for the "wet clutch" & it always worked just fine. I switched to Red Line synthetic, but I still use 75W90 with the proper additives for THAT application - which motor oils do not have.

If you're old enough, you may remember when if you had a "Pos-I-Traction" type limited-slip differential in your car or truck you had to add a little bottle of additive anytime you changed the differential fluid, or the clutches in the differential wouldn't work right & you wouldn't have "Posi" - the right rear tire would spin, but not both of them equally 'cause the clutches didn't lock up. That additive is now mixed in with several gear lubes - but not all of them - have to read the label.

The additive allows wet clutches to slip a certain amount when they're supposed to, but not slip when they're not supposed to & oils with other "friction modifiers" can make your clutch slip. So while H-D says it's OK to use motor oil in the primary/trans., you'll never see the day I do. Any quality 75W90 gear oil with the proper additive for the application will work fine - conventional or synthetic, your choice. Synthetic costs more but it lasts longer too - just have to calculate the cost/benefit ratio.

Try this

H-D sells a new oil for the primary/trans for the Sporty and big twin. Its labeled Formula+. I dont run the motor oils in the trans in mine either. I have found that the formula+ tends to quiet the gear noise in the trannys and is rated to 10k mile change interval in the Sporty. Big Twin is 20k in the trans,10K in the primary.

hddoc94565 clarified a point I didn't that relates to the original question - yes, Sportsters do use the same fluid for primary & transmission.

The Sportster's engine & trans. is "unit construction" & as of 2004, the "trap door" for the trans. is gone, so you have to split the cases to even get to the trans. The trans. on BT's is a separate unit & can (should) use a different fluid, specific to that application. But as hddoc pointed out, the same primary fluid can be used in both a Sportster & a BT, both of which have a "wet clutch".

Don't Know What They Did After O3, I Asked The Same ? Last Time I Was Changing Mine, I Did Away With The Hd Sportytrans Fluid For Gear Oil, Works Real Good Nice And Smooth As Long As I Don't Over Fill It.