The mechanic means the paper gaskets for the rear timing covers, they tend to leak or break over time and cause major oil pressure problems. I bet he just suggests you fix/upgrade them before modding as a
just in case since sometimes they break other times they don't, complete hit and miss. This is a very
very well documented problem on the earlier HR motors, I cant believe this doesn't ring a bell around here? The upgraded gaskets came out in 2011ish and later models and were metal instead of paper.
https://www.myg37.com/forums/g37-cou...erience-4.html