I am running a electrical circuit to a range that requires 50 amp serv I have run my conduit is it OK to use #6 thhn for power and#10thhn for my neutral or ground I was told I could use one gauge lighter for grounds and neutrals??

Yes, according to table 250.122 (pg. 70-117) 60 amps uses a #10 equipment grounding conductor.

I usually use an #8 for Neutral?