Flush the freewheel out with an appropriate solvent, such as mineral spirits. Allow that to drain for a couple of days or blow it out with compressed air(two minutes). Then, using a very light transmission oil, allow that to drip in, from either side(it really doesn't matter). Drain or blow out the transmission fluid. Apply more transmission fluid and allow it to drain only. Wipe everything off, grease the freewheel threads, and install the thing. You're set.

That's the way I do it and have had perfect success so far. Hope this is a help.