Hiya Sam, Some tubes are naturally a bit porous anyway, best you pump it up first and see how long it takes to lose pressure.
Even with new tubes on, I need to add air every week or so (sometimes I don't check 'em for a while though)
I had 'butyl' tubes fitted on my XT years ago, and they held air better Haven't researched 'butyl' yoo much, I more or less took the guys at the bike shop word for it, but I went there often.
Another thing I learned/was told at a young age, is to never use levers to refit a tyre (potential pinching)
- Use a rubber mallet with care, it shouldn't take much.