I'll take the first stab at answering, and the rest of the collective Textkit brain can correct or improve my answer where necessary!
quo utaris ipse is what newer grammars are calling an "autonomous relative clause." The whole relative clause is the object of
communicare:
{quo utaris ipse} communicare cum pluribus, "to share with others what you use yourself."
Older grammars would say, "Supply
id before
communicare," which you can do if that helps you to understand it better. (We recently had a discussion about this on another thread, in case you didn't see it:
viewtopic.php?f=3&t=10304)
For the sentence as a whole, I think it's best described as a mixed condition. The first part,
Si gulae temperes, is like a future less vivid: "If you should moderate your appetite / if you were to control your appetite." The second part,
non est onerosum ... communicare, states the rest as what is generally true (as in a neutral condition). That leaves the question, "Why is
utaris subjunctive?" Others may have a better answer; I think maybe it's subjunctive of characteristic: "the sort of thing that you use." Or else it's picking up the idea of a future less vivid, even though it's technically not the apodosis of the condition.
