Global Hobby has quite a few of their product manuals online http://www.globalhobby.com/review/reviews.asp

This is the direct link to the Magnum .91 four stroke manual http://media.hobbypeople.net/manual/210970.pdf On page 3 they specifically recommend castor oil, mixed with synthetic. There is definitely nothing wrong with using pure castor oil as your lubricant.

Page 8 discusses adjusting the valves and has a drawing. All brands are pretty similar so this manual can be a reference for any of them.