Alongar vs. prolongar.
My hopes of figuring out a pattern for o->ue stem changing -ar verbs are dwindling since alongar has a stem change and prolongar does not and there seems to be no logical reason why.
I don't think my lists are complete, but there seems to be more stem changing -ar verbs than -er verbs. The stem changing -er verbs for both o->ue and e->ie seem to be consistently restricted to a small set of letter combinations, whereas the -ar verbs do not.
Hmmm...excellent question... As I try to think of other o/u-ue stem changers that are -ar verbs, not many come to mind: tostar, trocar, tronar, forzar, colgar, jugar, and avergonzar. I tend to think of the ue stem changers as being mostly -er/ -ir infinitives, although now that I think of it, I've never sat down and made a comparative list! I suppose it's like a lot of language learning - initially memorize, or if you are fortunate enough to live in a Spanish speaking area, things just start to sound right or wrong as you hear them more times.