..... how do i know if its an oil bath
If lots of oil drains out of the damper side when you unseat the shaft bolts. Although with some forks that's also a sign of blown internal seals, especially Rock Shox.
As a very general rule, there's only about 10-15ml in each lower leg for lubrication. The main exception is older & lower-spec Fox which use the same oil bath for damping and lubrication, these have around 160ml in the damper leg.
If it's purely for lubrication you can use a fairly thick oil, which will stick better to where it's needed, but in an OB damper thick oil won't flow well so you'll get a very heavy damping effect (slow fork in both compression and rebound), so you need to balance out with something a bit thinner.