Dan,

The only "seals" you have to worry about are the rubber o-rings that go on the high and low pressure lines where they enter into the firewall. On the other side of the firewall under the dash is where the evaporator is. I would install new o-rings while you are in there. These can be purchased through Nissan or through auto parts stores in various sizes.

The only oil in the system is the mineral oil that lubricates the A/C compressor and you should not need to add any at all if you are simply replacing the evaporator.

Keep in mind that *any* time you open the A/C system to the atmosphere, such as when replacing a compressor, a line, or the evaporator, etc..., you will need to install a new drier/receiver. This the aluminum cannister located right next to the battery. Did they replace the drier when they fixed your A/C? If not, then there will be moisture trapped in the system, which makes the A/C not as cold as it should be.

Good luck figuring out your problem.

Later,