Mazda 6 blue white smoke out of exhaust and useing oil told it could be valve stem seals

Could be but........there's a few questions to be asked,did you ever run it without coolant?
has it ever overheated,or stayed in the red zone?
See stem steals are easy,my worry is these engines can loose hardness/temper in the rings if they have overheated then your just throwing oil down the bores.
A vacuum gauge idle reading can give a good indication of valves wear,fluctuations show up on the worn cyls,and I'd give the bores a cylinder leakage test to narrow it down.

check for oil leaks in engine compartment, get it compression tested

a quick way to check if its valve stem seals or oil rings!

let the vehicle idle for about 5 minutes.
if the vehicle is smoking on idle then your oil rings are worn. if there is no smoke on idle and when you give it a ref (meaning push down once on the accelerator pedal) and it starts to smoke. then valve stem seal are worn. my advice if the vehicle has high mileage. have the engine redone.