Hopefully this isn't still a problem and the truck keeps doing work for you. But if you still have this issue, please note that the older Hyster and Yale ICE products used a wet sump to lubricate the gear pump's drive chain. That being said, the starters in such trucks use an SAE no.1 flange and are sealed units. The starter should keep out oil on it's own. However, both mating surfaces (on the starter and transmission) are to be sealed using Loctite 515 gasket maker (Loctite is OEM, but similar is fine) and let it set for a bit before use to prevent leakage around the starter. If you have sealed it and the starter itself is leaking, you have a faulty seal in the flange of the starter. I'm not sure what model of truck you have, but typical one for trucks of that age is Hyster PN-1383240 (Denso PN-228000-5861).