Madbull adapter will do fine. 100% silly oil is perfect.
Depends where the leak is, it's not difficult. Preventative maintenance is best. Keep some propane in mag to store as well as lube regularly. Avoid storing dry for prolonged periods. A silly oil bath will sometimes revive the dried orings but it's better to avoid getting to that stage.
It's best to game it at 15C+ any lower and you'll see a significant drop in performance as well as capacity. GBB's are used in colder temps but with the assumption that it'll not shoot more than a few rounds.
Keep the GBB clean, lubed and fed with good BB's and all is good!