It says right on the Eaton link posted previously not to use any oil other than supercharger oil.
"New oil can be obtained from any GM or Ford service parts facility, or from one of the Eaton aftermarket supercharger partner companies or remanufacturers. Please make sure you do NOT use any other type of oil, including synthetic motor oils. These will cause failure."
It is surprising that people are still using motor oil after all these years just because somebody posted a thread about it. If Eaton, the manufacturer of the supercharger, says anything besides supercharger oil can damage the unit, why would you question it?
Use supercharger oil.
Regular motor oil DOES NOT have the correct specifications as the OEM supercharger oil. Some guys may get by using it, but the properties are not the same. As the reps at Eaton will tell you, it is not the correct fluid.
The OEM supercharger oil is made by a company called Nye. Here is everything you need to know about it:
http://productsearch.nyelubricants.com/pdf/TDS_English_SYNTHETIC OIL 605.pdf
The Ford AND GM oil are the same, so don't cut corners - just by two 4oz. bottles from your local GM dealer (much cheaper than Ford) and do it right!!
Edit: By the way, don't mix supercharger oil and motor oil - different base, different properties, and different viscosity!
The reason Eaton recommends NOT using a conventional or synthetic oil in the blowers is because the GM/Ford/ Eaton supercharger "oil" is a glycol based high temp lube. Look at the MSDS sheet. It is a PAG oil.
Glycol based oils/lubes are generally not compatible or misible with hydrocarbon oils. They do not mix.
Residual amounts of the original fluid can be a problem if they are not completely cleaned/flushed out first before using a hydrocarbon based oil.
http://www.dow.com/ucon/osp/pdf/L104_22_DOW_Lube_Tech_v2.PDF
If you have the snout off and can clean it out well with brake clean or something else that will leave no residue you should be ok.
Assuming you have selected an appropriate high performance high temperature synthetic oil replacement, that is also the correct viscosity for the application's rpm.
You can also do a simple drain and fill if you do it twice, this will get most of the PAG glycol based lube out.
We recommend Royal Purple HPS 5W-30 (part #31530, quart bottle) or Royal Purple XPR 5W-30 (part #01021, quart bottle). These oils are not regular motor oils, they both provide between three and four times the film strength of other oils including the original PAG factory fill Eaton oil, and will live in this high temperature environment.