What should the correct low-side pressure be for a retrofit with the engine at idle?

My honey-pot-pie had its A/C converted over to R134a six years ago by a shop. I bought the car last August and I would say that it performed better than luke cool. This spring I didn't get any cold air and it was running ~5 degrees over ambient.

I had assumed that I was running freon still and borrowed a coworker's gauges. I was excited when I realized they wouldn't work. I already recharged my 2000 Chrysler so it would be really easy to drop a new can in the Benz. I've done that and now it's blowing colder (beau cool?) but I think I could add another.

When I added the first can yesterday the gauge read a nice 35 psi at idle on the low side. This morning it read just over 25 at idle and then I had the wife rev her up to 2000 rpms and it dropped to 20 psi. I'm not so sure that there was leaking from yesterday to today but I'll take more readings tonight, before and after the trip home. Initially, when I was charging the system, there was high pitched squeal after the engine was turned off but that's pretty much stopped.

So, let's say I add more R134a -- I should probably add more oil too. Is it safe to assume that the shop used ester oil? It's my understanding that PAG oil is only used in systems designed for R134a.

Also, where in the hell is the high pressure port?

My hopes are that I can get one more summer out of this system. Next year I'll tackle replacing the compressor/receiver/dried/expansion valve.

Side question -- why does the expansion valve need to be replaced every time you purge/vacuum/recharge? I can see replacing the o-rings but why the valve?

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Own: '85 300TD Turbo - 171K miles
Owned: '81 380 SEL - 180K miles