When I ran out a few years ago I bought some "Rail Snot....Nothings Slicker" for about $10, good sized tube, works great, still have about half left. No idea on homemade substitute, but I'm sure someone on the site does.
Remember.... they're full time deer......we're part time hunters!
I would be careful what you put on your rails because it will be on your string as well. I just bought some Scorpion Venom lube to replace the no name stuff that came with the crossbow, the Lancaster Archery shop recommends it.
When I bought my crossbow, the Tenpoint/Horton rep told me any light oil would do, there was nothing magic about the stuff they market for crossbows. To that end, I have not tried anything else yet but as soon as I use or loose the oil that came with my crossbow, I will probably try Ballistol.