Going to be replacing the leaking service valve on the low side port soon. Going to vacuum the system. Do I need to add oil as well as the R134?

 

You should only add oil if you are replacing a major component that would hold a considerable amount of oil (compressor, condenser, evaporator, or lines). If the system has already leaked down or you have had it evacuated and are only replacing the valve, you should not need to add any oil.

 

After posting the related threads seem to have my answer. Which is no.

Next question. Compressor has a little rattle when running. Would a little oil help that or is it probably in the clutch.

 

It is possibly the clutch. Should be easy enough to check by removing the belt and turning the pulley the belt rides on by hand. Any grittiness or play and the bearing is on its way out.