i have a 1989 gmc r3500 with a 454.my ac line started to leaked and it leaked out all the r12.i was wanting to know whats the proper way to change it to r134a?do i just need to get the port adapters and fill it with 80 perfect r134a and add the new pag oil or will the old r12 oil need to be drained?

You should get a conversion kit. Need to replace the components with proper o-rings. Evac system for a couple hours and charge with correct oil and 134.

all lines are being replaced since those are what leaked.also replacing receiver drier.will drain the old oil out of the compressor and old dryer and measure it and put in the same amount of the new oil.will that be ok?also i heard i should use ester oil instead of pag since there could be some mineral oil left and ester oil is better around it.

You lost a bit of oil through the leak. No telling how much. You should be close if you add an extra ounce.

been researching online and it says total system capacity is 8oz.would i be ok to try to get out as much mineral oil as possible and just add 8 oz of ester oil?

That would work.