You're suppose to use a much lighter weight oil for the adaptor: http://www.airsoftparts.ca/store2/in...roducts_id=623
Mags are not hard to fix, it really depends on your mechanical aptitude.
Yes, store the mags with a bit of gas inside.
It varies from model to model, brand to brand. My own rule of thumb is to avoid using GBBs below 10C for plastic slides, 15C for metal slides. It's not that they stop working right at those temperatures, it's that they become unreliable, undependable and a liability during play.
Always wear eye protection, even when plinking. Make sure there is not a BB in the chamber before storing the gun.