what type oil is best used when doing a comp check
It's not critical.. First test without oil, then with. Lots of difference indicates problems with the piston rings, bores or pistons. Bad compression that doesn't improve with oil in the spark plug holes indicates leaking valves.
(Message edited by Roger K on June 17, 2015)
A tablespoon of your favorite motor oil is used to see if it increases a dead cylinder. This will tell you that you have shot rings. A compression check is done without adding any oil.
Normally you simply open the throttle, remove all four plugs, and do a compression check by cranking each cylinder the same number of times until the gage reaches maximum on the best cylinder. The difference between cylinders is what you are looking for, more than the exact number.
A difference of 20% from any cylinder to the average is reason to remove the head to investigate.