Yeah, under no circumstances put WD40 on any part of an AEG when to do so might get it on the hop rubber, hop unit, air seal nozzle, or gearbox - easiest just to avoid it completely.
You can use silicon oil to clean a barrel, but it's sort of like washing dishes with just water - it takes a lot more elbow grease and you wont get them squeaky clean. You can use just water too, but the same thing applies. Isopropyl alcohol is cheap, cuts grease easily, and dissolves dirt readily. If you use
these pads you can wrap them around one of those
plastic barrel cleaning/clearing rods - I cut the folded pad in half and use 1 layer in 2 stages. If I'm in a hurry, like it's the night before a skirmish day and I'm not sure if I did clean it after last use, I dry the barrel and rubber out using some SA80 barrel cleaning ribbon* wrapped around the rod, otherwise I let it air dry.
*Which you can see in
this drastically overpriced fleabay listing. It crops up on its own occasionally, but you could use a piece of lint-free medical gauze just as well.
A few times I've given them a good 'scrub' using soapy water, which I then rinsed out with water several times, dried and then finally swabbed with alcohol to make sure that there was no residue of soap left behind. The dirt which comes out when I just use ribbon/swabs soaked in alcohol is always black, but the ribbon I've used with soapy water has had a slightly brown tinge to the stain left on it, so I think that it is getting something out which alcohol alone is leaving behind.
As UTJ said, I wouldn't get oil on the hop rubber - it's supposed to be sticky so it grips the top of each BB as they pass. Micro droplets of oil and/or grease will continuously coat the rubber through the barrel window, because they will get blown out along with that portion of air which has been sucked through the gearbox and also enter the airflow during every back stroke of a vented piston head, assuming there is grease/oil on the O-ring. However you choose to clean the rubber, in the hop unit, or after some degree of disassembly, getting rid of grease/oil
is the point.
I have heard that soaking silicone rubber parts in brake fluid makes them a bit stickier, but I've never tried it. Just from the general smell of the stuff though, I wouldn't be surprised if it could deform it too.
Any steel parts of a gun should always be oiled or lightly greased after getting wet, to prevent rust obviously, and it wouldn't hurt to do the same on any wooden parts too, to stop waterborn microbes/spores getting into the wood through any scratches. Eventually it's probably a good idea to disassemble guns and clean off any crap which has collected inside and the lubrication inside the gearbox can't last forever, particularly that which helps the piston O-ring seal.
