Possibly condensation over time. To fix drain, flush and refill with new ATF. Other possibility is that the trans has been submerged some time in the past, as was the case with a boat I once bought. I initially did as above but later stripped the trans and found some minor pitting on the bearings which I replaced. The trans was operating OK but I stripped it down primarily to change the ratio and the bearing replacement was incidental to this. Slipping may be due to the position of the f/r lever. You could check that the travel is correct and the gears are engaging correctly. Good luck,