Rancher 350 Oil In Breather Box
Usually, this is just drain down off the air filter, but....
....if it was burped up from the breather, it could be several things. Later models got a baffle in the crankcase in front of the breather, that helps prevent oil splashing up into the breather tube. You may have an early model without this? (Mine is an 00, and the very first one my dealer sold, so it has no baffle, and I have never had a problem with this! I did however have a 2wd 01, that did puke a lot of oil out the breather.)
Or, you could have worn rings (very possible given its age) and are passing compression into the combustion chamber, that then gets vented out the breather...and carries oil with it. A compression and leak down test can diagnose that.