Seals are usually natural rubber because its cheap. You can upgrade to buna nitrile though.
Gas gun you want air tool oil. Aegs you want a grease.
BioBBs are fine. Quality is everything and at $30 for 2500 of the best rounds im sure you wont complain lol
There's no "optimal" barrel length in terms of BB performance at range....
The major difference is that we control the volume and air pressure. Not necessarily separately, but because the air pressure changes, the optimal length also changes.
Ive got a 300mm barrel in my M14 and I can hit someone within 2 shots at 250ft.
Same deal with my Mk18 and at 250ft the only thing affecting accuracy is the wind.
In the 300ft+ range it takes more luck than skill with the wind to hit someone but ive gotten a kill at 350ft before and other people have gotten in the 350-400 range with bolt actions and heavy ammo (.36g and above)
At 400-420fps you'll be using .28 to .32g
Pistols can be finicky; my usp worked best on .32s, my m&p works best on .25s
But thats usually the hop rubber not being able to handle heavier ammo