Well, here's my 2 cents worth on the subject.
In my opinion, and it is just my opinion, There are a couple of different things I am personally concerned with that are going on within the primary.
1. you have a clutchpac that is really not a whole lot different than the clutchplates found within an automatic transmission.
2. you have a roller chain that requires a lubrication that is more cohesive to prevent premature wear.
3 you have a compensator and sprocket teeth that require the coating of lubrication to prevent heat build-up and excessive wear.
Personally, I like the heaver oils because I feel they provide better overall protection for the majority of what's in there.