RE: Do 4stroke engines need to use 4stroke fuel?
This is from Tower Hobbies on the OS .91 4-stroke page:
" NOTE: Certain four-cycle fuels cannot be used due to low oil content."
Some 4-strokes like a little more nitro but they all need good lubrication. Just choose a fuel that meets the recommendation in the owners manual and you will be fine. It does not have to say "4-stroke" on the label.