Just wanted to comment on recent experience.  A bit handy so I do most of my own work on everything I have especially car boat and bike servicing.  Bought a twin cam recently and went to dealer to get some oils.  Figured for my first go on this ike I would use what the dealer recommended.  He gave me 6 quarts of 20/50 to use in Engine, trans and primary.  I didnt get it but I left with the oil.

I was a car dealer mechanic a generation ago and i just couldnt put the 20/50 in my trans or primary.  Didnt seem right. 20/50 full synthetic in the engine No Problem.  I use full synthetic in all my engines nowadays.  Bought some Nulon 75/90 Semi Synthetic for the trans and used it.  Less Clunk and smoother shifts.  Happy with that decsion.  Havent done the Primary yet but wondering if anyone has a better idea than Motor oil for a Clutch Pack Assembly.  Seems like if its a wet clutch it would be better with Automatic Trans fluid.  Auto transmissions have wet clutch packs and have a similar operation.  Have any of you guys used other fluids in your primary's ?

G'day Jersey, I use Mobil 1 V Twin full synthetic in all the holes in my 99 twin cam.

https://mobiloil.com/en/motor-oils/mobil-1/mobil-1-v-twin-motorcycle-oil

atf is good for da clutch but maybe not to great for da chain i have use it heaps in my own bike's & still do in my race bike pri ,but for normay work i use HPR 40 Premium  Penrite it has no friction modify in it  , good for the chain & as thick as you would go for a clutch pack no drag & all being using in fellas bike's for years , it get pretty hot in there esp with chrome outer covers etc , iv'e seen some oils come out brown from hot use in da pri ,   no chain wear  with HPR 40    i use it in 1984 & up Sporty's also

Yes I'm a strong non believer of using a motor engine oil in a gearbox! I use an industrial mobile 634 synthetic gear oil in my tranny and it is quiet as a mouse.
I've played around with different primary oils, ran ATF for a long time because of the same thoughts as you, would be best for the wet clutch, I had no problems with it but started getting concerned that maybe it was a bit light, not oily enough for the chain and compensator seals etc.
I'm now running a four stroke motorcycle engine gearbox oil (in the primary only), one that is made for an all in one solution .

I would have thought this has been covered in numerous "Which oil is best" threads.

While I don't believe that SYN3 is the best oil anyway it is safe to use in the primary. It doesn't have friction modifiers so is fine for the clutch pack. But personally I just wouldn't use it anyway. At present I'm running Penrite (same as Daddyracer56) in my engine, the older style ATF fluid in my primary and a proper gearbox oil in the trans. I don't know the grade of oil in there as my engine builder put it in when I had the 95" done, Though I suspect it is a heavier grade g'box oil.

I changed to putting ATF in the primary on the advice of a local guy who is not only a HD mechanic but has been racing HD's in one form or another for nearly 30 years and he swears by it. He still runs it in a Dyna that is turbo'd so it must be good enough. In the next few weeks, when i get of my arse and change the bloody oil, I'm changing over to mobil1 V-twin oil for the engine. But for now i'll stick with the primary and trans as they are.

Thanks Guys. Some great info. I'm glad I'm not alone and other guys are trying different things. Some good points on the ATF being a bit light for the chain. I still like the idea of the ATF and the 4 stroke Motorcycle oil suggestion. I'll be looking for a full Synthetic for the tranny next time as the synthetics handle the heat better and the twin Cams cases get pretty hot. I read somewhere of someone using 85/140 in the tranny as well. Maybe next time. For now though going up to the 75/90 gear oil in the tranny made a significant difference for the better in my opinion.