Looking to retrofit my 1990 c1500 from r12 to r134a. I already have the fittings installed and the system has leaked out a long time ago. I pulled a vacuum for 30 minutes and it held at 28inches of mercury for 30 minutes. My question is what type of oil do i use after flushing the evaporator and hoses and how much. Do i use PAG or ESTER. Also i think i need different pressure switches as r134 runs at different pressures. Any advice would be great.