The only oil additive that might not be snake oil is Marvel Mystery Oil. I have never used it but have met some old timers that have sworn by it.

 

As far as the A/C goes, replace the receiver drier (its saturated and is useless). You need to pull 30 in/Hg on your A/C system to get the moisture to boil off. Then it should hold that 30 inches for 5-10 minutes with no vacuum loss. This is the most important part. If it doesn't hold the vacuum, it has a leak in the system somewhere. Find it and fix it. Then vac down again and if the vacuum holds, recharge with the porper amount of R-134/PAG oil.

20 inches is iffy. Would be a waste of time/labor if it didnt get correct pressures on the manifild and you had to recover/vac/recharge again.