The compressor is just a pump to move fluid through the system. So long as you have the correct safeguards in place to prevent damage to the compressor such as high pressure/low pressure cut-out switches in the system I personally wouldnt worry about it.
Keep the system R134, dont try to run R12/Freeze12 in it as the oils in the systems are different and wont be carried by the refrigerant. (kinda like not having a pickup tube on your oil pump in your engine) This would starve the compressor for oil and over a period of time would kill it in a bad way.
Also make sure you use the largest most efficient condensor you can fit. (Radiator looking coil that goes in front of the engine radiator) This will make a big difference on efficiency especially at slow speeds.
Chris
