The conversion was as easy as swapping the inlet valve for the correct hpa adapter. Yes green gas does have silicone oil in it to help keep O rings lubricated . So far I've not had any issues with the seals in the drum mag though .
The mag capacity thing does take some getting used to . When I run my L119's I carry 5 stanag mags and a speed loader , I can get 2 plus BB loads from a gas fill on those , so that gives me something in the 300 round total for an hour game which I find enough for my play style . It's all working in close to the other team , picking your shots and double taps !
The MP5 mags would need a refill of gas too , so when I use those I carry the speed loader and a can of gas . I have 4 stick mags . Just re fill during quiet moments or when re spawning . Same when I use my MP7 .
The Apache certainly is loads of fun , I like the 3 round burst on it .
I guess it depends on how much you enjoy the mag changes , if you aren't fussed then the drum might be the way to go , I'd still carry spare gas for it and give it a quick top off now and then . If the mag changes are a big part of the fun I'd stick to the stick mags . However if you want to run it in the winter the drum with an hpa adapter will work better given the size of the gas capacity in the stick mags.