Hi Paul -

The larger hose is always the vacuum side of the a/c system. When you say service valve, are you refering to the schraeder valve on the manifold hose? If so, yes, oil can be added through the valve. If you have the system de-pressurized, I suppose you could remove the valve core and add oil, but it sounds rather tedious. On my 124 (which is a diesel, and hence has a different manifold setup) I just undo the suction hose to manifold connection. It's just in front of the forward firewall, where the suction side passes through the firewall. Then you can just pour in the appropriate quantity of oil. You'll need to replace the o-ring in this case, though.

There are also tools available for injecting oil into a pressurized system. Basically a small cylinder which is filled with PAG, mineral oil, whatever, and connected to the system. The opposite end of the cylinder has a connection for refrigerant, and the pressure of the refrigerant is used to push the oil into the system.

And of course there used to be little cans of half oil/half refrigerant which could just be added to a system. Don't think I've seen any of these since R-12 lost its popularity.

- JimY