Oil is a lubricant. If you mean oil vs grease however, there is a difference. Oil is generally used on more dynamic parts, such as the ram on a cocker or an Ego. Greases are used on regulators, spool valve markers, etc. The manual for any gun should tell you what lubricant to use and where. Cockers and tippys should be perfectly good on oil. Just pour a couple drops in the ASA before you play. You will want to disassemble and clean your regulators periodically though.
If in doubt refer to the stickies on the page for your gun here, they usually have some kind of guide to marker maintenance.
Northern Arizona University Paintball
Current Markers: PE Director's Cut 2 Autcocker, PE Aurora SL74, CCM S6.
Generally, any stacked tube poppet (Egos, Timmys, Legends, Spyders) use oil on their rammers. I don't quite remember with the Timmys, but I think some of them use Dow55? Spool valves generally use Dow33 or Dow55 (for Marqs - inline poppets).
For regulators always use Dow33. These are just general rules so they may vary depending on the gun. The best thing you can do is read your manual.