Oil vacuum pumps last longer and usually pull higher vacuum. You don't really need the level of vacuum needed for refrigeration when vacuum veneering or holding parts, but a high cfm will overcome small leaks and pull the air out of your vacuum bag faster. Air is being pulled out, so how is oil to get into your vacuum bag or chuck, unless the pump suddenly stops and leaks backwards. I am more concerned with pulling sawdust and debris into my vacuum pump than oil going backward from the pump to my project, so I use a filter in the vacuum line to prevent this. On the exhaust coming out of the vacuum pump, I also have a filter. This one is one of those orange disposable filters like used for spray painting. I don't replace it after every use, but do replace it frequently. It keeps the oil mist out of the shop air very well.
Charley