Wall mart sells Brass Eagle paintball Silicon oil and its something I barely use.. I havent greased my mechbox since I put it together.
I only grease the inside of the Oring on the piston head and turn the oring a bit to get it fully around it.. then take a bit and mid way in the cylinder I push a bit of grease around that so its a full circle. I grease the ribs on the piston and thats it.. I did this all to my M14 and it ran 1.5years at 425fps... without ever openning it again.