There is so much confusing information that I am not sure what is appropriate. Some folks say that Richie used mineral spirits before 2001 but it had a higher flash point. Richie recommends their Isopar product but 1; it is expensive and 2; there seems to be two types of Isopar and one must not be substituted for the other.
Then again, there are folks who used mineral oil but some said that the oil they used made the compass respond too slowly.
OK, I looked up Richie's FAQs:
What type of fluid is in my Ritchie Magnetic Compass?
Until 2001, all Ritchie compasses (see exception) were filled with Odorless Mineral Spirits (100% Paraffin based). Ritchie now uses a fluid called Isopar L that has similar properties but some added benefits over Odorless Mineral Spirits.
Exception: Some old style Flat Top Compasses used a mix of denatured alcohol and water. This process was stopped in the 1960s. If you have a Flat Top Compass, it is recommended that you test the fluid to see if it is Odorless Mineral Spirits or an alcohol/water mix.
So it seems that the old and new compasses don't use oil..