Definitely go with the mineral oil. I had a fun time finding someone who had the correct mineral oil but I wound up with the same stuff you have there in the 32oz bottle. For leak testing, I would use propane because it is cheap and I have it ready. It's probably not dry and I don't know if a leak detector will sniff it but an ounce or so of R12 mixed in should do it. I'll probably catch hell for that suggestion but then again, some people run propane in their A/C systems and get good results. Guys, spare me the lecture, I know A-Z argument about propane but I am only suggesting he leak test with it given he has R12 which is the best option.

I just re-did my A/C in the 300SD and it is much better than it ever was with R134a. It's not blowing 40F at idle with 105F OAT but I would love to see an MB car that is. I also need to install a larger condenser fan which will help with idle vent temps. I'm not sure where in Dallas you are but if you need help, let me know.

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge