R12/R134a gauge set question

[erubin]

I own a Mastercool gauge set marketed for charging and testing R12 and R134a. Up till now i've only used it on R134a systems.

I plan on working on my friends R12 equipped vehicle by leaving off the R134a couplers.

In reading Mastercools instruction sheet it says: "Do not use the gauge set on refrigerant systems containing mineral based oils". Does this mean i can't use them since R12 typically (?) uses mineral oil. Why would they market it for R12 and R134a?

Is the risk in damaging my gauges or is it in contaminating between systems? the gauges are used infrequently.

i remember reading somewhere that hoses will absorb the mineral oil but I don't plan on using them much on R12 systems and will never load mineral oil directly through the hoses.