Really this depends on what is leaking and why.
For instance, a minor leak from a rear main seal way be sealed up with some stop leak product. There are new items like this on the market that really work well. Although I have never seen permanant results.

If you have a torn seal or broken out gasket, there is no cure. Time for a replacement.

Also, some seals, and sealants are not tolerant of certain fluids. For instance, the common, "blue" gasket maker isnt oil impervious, meaning that oil will soften and finally distort the sealant. Certain synthetic oils are harder on gasket sealers and some ruber seals as well. Most manufacturers are going with plastic seals as of late. Also anerobic sealants selan when they are NOT in the presence of air making them more relaiable. You should also look for oil impervious and oil resistant gasket sealants.

What trucks? What seals or gaskets leak? What oils/fluids are you using?

Doc.