I used it in my wifes TransAm rack and pinion with success, but I think it is more designed for rack and pinion leaks. The steering shaft seal isn't a hard repair, an hour at most I would think. I did mine myself and took my time with it since it was the first one I've ever done, took me about 2 hours.
It helps slow leaks from a seeping seal, like you're describing. It works fairly well.
Then again, the power steering on big trucks is one of the simplest hydraulic systems there is, and there's no exotic materials involved. You could pour 15w-40 in there and it would function just fine. It would also leak less, partially because of the higher fluid viscosity.
Just saw this thread. Lost power steering in my E350 at about 500,000 miles, replaced with Lucas and no more problems until I scrapped the van at 1,000,000 plus (not for steering). Useful to limp home with a blown differential as well