Don't get dragged into the 2stroke oil snobbery. Yam 2 strokes in stock trim will just about run on chip far, no im not joking. Big roller bearing mains, needle roller small ends, the engines are bomb proof if ridden/maintained right.
I have been using morris semi synth 2 stroke oil (fiver a litre from Churchill motors Pattishall) for years with no problems at all.
As for the seat I cannot help, last 350 I had was in the early 90's as a winter hack (yeah I know) cannot remember how to get the seat open.