Uh, the oil circulates with the refrigerant on the earlier type compressors too, no way are they oil tight and the service manuals clearly say that discharging refrigerant too fast pulls oil out of compressor with it to lose track of where you are amount wise. If you use PAG oil with the R134 you MUST get every last bit of the older mineral oil out, it can react with it to bubblegum up like jello and tear up the compressor. The official industry conversion number says the mineral oil must end up being 2% or less. If you use ester oil then you do not have to get 100% of the old oil out. The oil type can make you or break you there. There is no such thing as a 'dry' system, every a/c system I've ever worked on even back in the '60s required oil running mixed with the refrigerant. Not as much as the later ones or these with compressor turned on its' side but some oil is always in the freon, it lubes the reed valves to not overheat, the hottest parts of the compressor.
Dryer changes if left open to air 24 hrs. You CAN get by without changing but you must do a super vacuum job and a refrigerant sweep will help as well. Any o-rings are different material too but they'll go for a good while before failing. I vacuum my own stuff down using a simple 110 volt compressor out of a 5K BTU home a/c window unit. Maybe $10 worth of fittings on it to make it work.
R134 will not cool quite as well as R12 with R12 equipment used, you can make it better at idle with additional condenser cooling like an add on electric fan, the OEM generally doubles up the finning on R134 condensers to make up for the refrigerant difference. R12 highside of say 200psi will need to be around 300 psi with R134 to get the same amount of cool, that may show up problems with older hoses. R134 systems have high pressure switches that cut out around 450 psi, a good chunk higher than R12 since the normal running pressures are higher.
A good strong blower inside car helps a conversion as well, but an act of Congress getting one of those, the available ones you get anywhere OTC are nowhere near as powerful as they could be, the industry does not care either. I've long said that someone could step in there and create a killer market selling 'high-performance' blowers, people never grasp how much a high output there can increase the cooling of cars. You can freeze the evaporator solid and if the blower does not move max air the output will still s-u-c-k.
I converted cars there for a bit and they worked very well. My Tempos were converted 20 years ago and they still cool fine now. I hate modern swivel o-ring sealed systems though, they tend to leak down faster than the positive metal-to-metal connections the older cars had. Again, all they care about there is installing the a/c lines without using a tool on the assembly line at all. All done by hand only.