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).

The starters are OEM starters now. We got a cheap one first then spent the money for an OEM.

these starters have internal seals and are designed for these engines. Most likely you were getting a non-oem starter? Might be the ones you were getting were a cheap aftermarket brand?
I would use an OEM reman or take it to a starter rebuild shop and get it rebuilt there.

Now if the sump housing is building up too much pressure and blowing out the seals in the starter you might have to check the return line and filter and see if there are any restrictions.

If you want any model specific information on anything else we will need to know the FULL model number and serial number.

are you using OEM starters or are you using a comparable starter from a local parts house? the reason I ask is I was having this problem I crossed the Hyster starter part number with my local supply house came up with a number got it for 2/3rds the price of an OEM but every one of them leaked within a week of installing them 4 starters latter I got an OEM one and have not had one leak since. Just a thought