Having done a number of these, I can tell you the more oil removed, the better. If you can flush the system, that will be ideal. That's all there really is to it. Obviously the oil settles to the low points of the system. I briefly perused the rest of this thread, but didn't read it in depth, so I might repeat other posts.

In a working system that has a small leak instead of a catastrophic failure point, flushing isn't necessarily required as the system should be a properly functioning system. The Ester oil can handle the residual mineral oil, which was originally not thought to be the case in the early R134 conversion days. I'd stick with the ester oil, since it's less corrosive than PAG oil and less hygroscopic.

Many times the conversion is secondary, or a side-effect of another repair job. In other words, the "conversion" is only being done because the old compressor blew up or chucked junk into the system and eventually eroded the efficiency until someone said "I guess it's time to convert to R134 and fix the problem". Then, many times, the R134 conversion is blamed for ruining the system, when in fact the system was already one foot in the grave, the conversion being attempted as a band aid fix. If this is the case, flushing is mandatory.

Assuming your system is clean, functions, and is clear of debris, you may not need to flush it, but flushing it would be better. Get the oil out of the compressor, flush the system, reassemble with new o-rings. Replace the orifice tube (part of the Volvo kit), and replace the accumulator.

Another helpful addition to the system is placing a jumper wire in the electric condenser fan pressure switch harness, where the switch is located at the lower right frame rail in the little "L" or "J" shaped pipe (I do not remember exactly). What this does, is enable the condenser fan at all times that the blue snowflake switch in the passenger compartment is turned "on", whether the compressor is engaged. This easy modification keeps the high side pressure lower, which contributes to system longevity and warm weather performance. The temperature-to-pressure curve of R134a is significantly different than R12, and it can create very undesirable high side pressure. This is why overcharging or having a nonfunctional condenser fan is very bad. Also, making sure the condenser fins are absolutely clean of garbage will help.

When done, you should have a clean system, new O-rings, new orifice tube and accumulator, fresh Ester oil, and new fittings. Next, make sure you vacuum the system, which is absolutely mandatory. Charging by pressure is ideal, charging by weight is OK also. Obviously don't guesstimate and never overcharge an R134a system, because the performance falls off a cliff. The system's life span will also be shortened significantly.

You probably should adjust the pressostat for low side cutoff pressure. I don't remember the settings off the top of my head, but I'm almost sure I've posted them in other 134 a/c conversion posts, as I used to do a lot of this. On the 1991 model, there should still be a large pressostat switch which is adjustable. I believe 92 models had a non-adjustable switch but I'm not positive. The adjustment to R134-friendly values aids in system performance quite a bit.

A few last words: The 91-92 system usually performs well with R134a. I've done a number of conversions and they all performed well. I may have reviewed some things that you already knew, and if so, ignore them as you are familiar. Since I've done a lot of these, I've learned some things along the way that usually result in better performance, and I like to share them. A good R134a conversion usually results in good performance, while a bad R134a conversion can have very poor performance. I did also notice a mention of alternative refrigerants, and have known a couple to work well, but have not used any of them myself.