I need a little more info to help you.
1. Are you purchasing the equipment sight unseen like on an auction or in a big lot?
2. What are you wanting to accomplish? Is is a permanant fix, or do you just want to get them out of a 30 day warrenty?
3. Who is doing the reapirs? Is is your own employee, or is it an outside contractor.

I may be able to give you some help on how to quickly indentify leaks if you are doing some sort of inspection before you buy the equipment.

I also may be able to give you some tips on how to direct whoever is doing the repairs.

As far as the stop leak goes, I have not had any luck with it. If these trucks have been sitting for a while before purchase, they may leak worse because the seals and gaskets dry up on a truck that sits. They may be leaking just because they have been sitting idle are were not leaking when they were parked. If you want to try the stop leak, the truck will have to run for a period of time for it to soften the seals and even have a chance of working. I have NEVER spoken with any one that stopped a leak with any of this stop leak except on a cooling system. If anyone has had result from stop leak to fix oil leaks, I would like to hear about it.

Giving the make and models of the trucks you are purchasing would be helpful.

As "Doc" says above," What trucks? What seals or gaskets leak? What oils/fluids are you using?"

We can't give generic answers that will help you. Fixing a leaking 65 Chevy is a whole lot different that fixing a 2005 BMW.