I'm not an engine expert by any means but over time I have learnt a few of the series's. Eg in the O.S range the LA series is their cheaper non bal bearing engines while the FX series (also AX) is the more powerful ball bearing range. For Thundertiger their cheaper series is the GP range while the Pro is the ball bearing type.
With most manufactues if their engine name ends with a H eg OS50SX-H then it is a helicopter engine.
There are heaps of others but someone with more knowledge in this area ahould be able to help you out a bit more.