John, I have never seen a Compressor with that kinda pressure! 300 to 400 psi would kill about anything you used it on. Most switches I found were factory set to come on at 90 psi and below and to shut off at 125 psi. They all seemed to be adjustable, but not to that extreem. The major difference in the ones I found, other than cost, was some have an extra air line plumbed in to release pressure on the compressor when it reaches max PSI, a good idea for longevity of your compressor. That way it never has to start under pressure. The others are much more simple, on at 90 and off at 125. I got mine from Napa, about $22 if I remember. I have mine wired inside to a switch where I can have it on or off. This can be helpful when going down hill.... having the purge/drain line open so as to not build pressure, you can flip it on, making the compressor run, lowering engine RPM's, slowing your decent. Depending on the compressor used, it robs between 5 and 10 HP.
Plumbing the already circulated oil back thru the system doesn't seem like such a good idea to me. there will be a certain amount of moisture in the oil. A pre-oiler set up sounds to me to be the best option. I use 30 w in mine, and at the cost I don't think it would be cost affective to plumb it to re-circulate, no more than it will use. The oil seperator will give you an idea of how much you are using, and as to how often and how much to add back in. I have read of guys using slick 50 in them. leting the Slick 50 circulate in and then out in a circle on the intial install, coating all the internals at start up. then draining it and running no oil at all. I have not tried this myself, but have read it from several sources. Hope this helps
