Purchased an 80k mile 65 Galaxie 390 FE Motor, Not sure if motor was ever rebuilt. When got it ran great. No smoke. Took it to a local mechanic I have used before and he changed oil to synthetic added oil stabilizer and changed valve cover and intake gaskets. Now after warming up, I get a fair amount of smoke from road draft tube (no pcv). Didn't have any of that before. Could it be the synthetic oil? Looking for advice. Still runs fine. Smoke is annoying when stopped after it's warm.

 

What brand of synthetic did he use? I use Amsoil designed for older engines with flat tappets. Using a premium synthetic designed for newer engines can kill an older motor. Excessive blow by is caused by improper ring seal. I used to run Kendall GT-1 in my older engines. Just be aware you are most likely going to ruin your cam and tappets with the newer synthetic.