I think the best place to start would be the 2000 Ram Pickup Warranty Information Booklet. I think you will need to find something within the covers of this book to support your position. Here is the part of the booklet that mentions the Cummins engine warranty:
2. 6 Cummins Diesel Engine Limited Warranty
A. When It Begins If your truck is equipped with a Cummins Diesel Engine, the Cummins Diesel Engine Limited Warranty begins at the end of the 3/36 Basic Warranty period, and covers the cost of repairing those parts of your truck engine listed that are defective in materials, workmanship, or factory preparation.
B. Parts Covered for 5 Years or 100,000 Miles. In trucks equipped with a Cummins Diesel Engine, ONLY the following engine parts and components are covered for 5 years or 100,000 miles, whichever occurs first:
· cylinder block and all internal parts
· cylinder head assemblies
· core plugs fuel injection pump & injectors
· gaskets and seals for listed components
· intake and exhaust manifold
· oilpan
· oilpump
· timing gear drive belts and/or chains and cover
· turbocharger housing and internal parts
· valve covers
· water pump and housing
I don't see how you can argue that the oil pressure sending unit is covered.
The 2000 Ram Pickup Warranty Information Booklet also details other warranties: corrosion, noise emission, and emission performance.
Good luck in finding something that includes the oil pressure sending unit.