Regarding oil just measure the amount you get after draining the old accumulator. Old condenser typically stores about 30ml of oil. So the math would be 30ml + old accumulator oil. Pour fresh oil into the new accumulator's inlet port - one that's connected to the low-side port. Instant compressor damage might happen if you add oil into the accumulator's port that connects back to compressor. Total system oil for 940 is about 200ml. If you're sure the system has never been serviced before only add PAG oil because Volvo originally used that. If you're unsure only then you use Ester oil because it could comfortably mix with PAG.

Regarding oil viscosities use PAG46 only for new compressors, PAG100 for still working old compressors. For ester use POE100 (same viscosity as PAG100). Choose your oil wisely. Much like choosing engine oils. One way is to go for OE oils which there're lots of choice - only pick the one with right viscosity. Notable aftermarket brands are Emkarate (industrial grade) and Supercool. Most easily available at ebay. Other brands are still in grey area.

Amarin.