The fitting on a can of 134a is different from r12, so you will need to buy an adaptor.
I have 40 year old dehumidifiers that leaked out all the r12. I pulled a vacuum and filled with 134a. They work fine. I also installed an old thermostat from westinghouse a/c on the suction line, so if the suction line frosts up it turns compressor off. I just wish I hadn't pitched so many of the frost controls.
Then I removed all the electrical controls from dehumidifiers and put them under water to look for leaks, found none. So my leaks are extremely small. So far, 6 months later,my 2 dehumidifiers are working just fine.
I look at it this way, anything using r12 is very old so if you get a few more years out of it using 134a I'm not going to worry about the oil. The unit is on borrowed time.