As Hotfingrs has stated,you only need to do the "
oil trick" if you think you readings are low.......As long as all cylinders are close in thier readings you know the engine is fine,if you have one or more that are a lot lower than the rest,then try the
oil trick to help determine the problem......
Back in 1996,I built two 351-Ws,both basically the same,only diffrernce was the cam,one had a cam with lift around .500 and a fair amount of overlap,the other had a cam of around .425 lift with no overlap,the cylinder pressure was tested after both enignes had about 1000 miles on them and the difference between the two engine was around 75 psi,all cylinders were within 5% of each other on each engine..... Cam profile has a lot to do with cylinder pressure.......
I helped a buddy find a "miss" in his 351-W some years ago,we tried everything and could not find it,finally I suggested a compression test,7 cylinders were very good,one had only 20 psi,tried the
oil trick,it stayed at 20 psi,when we pulled that cylinder head we found a burnt valve on that particular cylinder......the test saved us from tearing down the whole engine and/or pulling the wrong head......it is a useful tool in checking out an engine......
David