I got to thinking - If the 1981 manual has it in it, so should the 86 Trooper factory service manual. Sure enough, I found it. It is slightly different so here it is:

http://www.atis.net/~yostsw/Clip2.jpg
Changed this to a link because of its size -- Randii

Basically the same but the plumbing in the cooler is different and the pressure values are different. The regulater in the block feeds the oil pipes only. The tips of the oil pipes are a bit higher than the cooler base, I believe, so I don't think that if the regulator for them was leaking the leak would drain the cooler/filter, or even the galley.

So I agree - I think it has to be the oil cooler valve. The call it a regulator in the diagram but a cooler valve in the Isuzu parts book I have.

Thanks again!

NCDiesel