Robinair Vacuum Pump and engine oil.

I am using a Robinair vacuum pump to evacuate a system that had been
open for quite a while. I ran it for an hour, and the moisture
content pulled into the vacuum pump turned the oil to a messy emulsion. I
want to run it until everything is clear.

Will use of engine oil ( 10-30 or such) have any harmful effect on the
pump? Robinair says use only their oil ($6/qt). My pump is strong,
but very old, and way out of warranty. A less expensive generic pump
oil is also an option.

Thanks

I wouldn't take a chance on using the engine oil. If the Robinair oil is only $6/qt, I would just be safe and use their product in your vacuum pump. A generic brand may be good option as well. After all, it is only 6 dollars. Good luck.