I was looking at some kits on eBay for adding some R-12 to my system. It definitely needs a charge because now that it's hot, the air isn't blowing cold anymore.

There are various kits on eBay that sell 12-oz R-12 cans along with a can of oil for R-12 systems. They include the can tap with a hose that ends with a schroeder valve. Some kits also have a pressure gauge.

A few questions:

1) how much of that oil should I put into the system?
2) some kits sell a pressure gauge inline with the filling hose. which pressure readings am i looking for to know that the system has enough R-12 added?
3) the instructions say to use a low-side port for filling. i've been told that on the 300SD, the low-side port is located up front behind the grille next to the radiator... can someone verify that please? there's another port that's by the firewall - is that the high-side service port?