This is not a model-specific question:

In r134a systems running PAG oil:

When evacuating the system to replace components, it is possible to get a lot of oil out of the system if the High side is opened up. (Yes, I have recovery cylinders, and everything).

Given that PAG absorbs moisture over time, and may get dirty in operation, is it desirable to take steps to ensure that the maximum possible amount of existing oil in the system is replenished?

-Or, is this not important when the receiver/dryer/accumulator is being replaced. Will the dessicant and filter make the oil "good as new"?

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1986 300E 5-Speed 240k mi.

As long as the system is not left open and only opened long enough to repair a leak or replace an offending part, putting in the same amount of lost oil before evacuation should get the job done.

Two points though; with PAG there are three different viscosities, make sure you use the correct one, and replace only the amount that was lost, don't overfill.

PAG is hygroscopic as heck. Get the system repaired and put in the PAG at the last minute. Take the top off the PAG can, pour out the measured amount you need, immediately cap the can, pour in the oil and seal it up. Pumping down an extra amount of time is advisable. If you have a vac pump or recycle machine that will allow you to evacuate overnight it won't hurt.

Good luck,