As far as the rockers on the OS 91, you should be fine as they are oiled up the pushrod tubes. I sure would use an after run oil in it, and my preference would be Marvel Air Tool oil, or Rislone. I have used both, and they both do well. I would not use WD-40 in any engine, as it has a bad habit of building up a residue on the cyl. walls that is real hard to clean off. I usually give mine a shot of rislone in the breather nipple after a days flying, and flip it a few times to distribute the oil. There are other after run oils that work well also, but my experience has been with these two.
The Rislone I use is the one in the quart bottle that is used as an additive, not the concentrate. Marvel Mystery oil is okay for short term, but no good for storage as it will dry out and leave your engine stuck with a red residue. Just about any air tool oil should work and not dry out.