The "diesel rated oil" you speak of is actually correctly called "universal" grade. There is no difference in the "thickness", the difference is the additive formula.
There are many oils that carry a C (compression ignited) rating, but are not specifically for diesel engines. The universal grade has additives that basically have more detergents which can deal with soot dispersal, something that is not an issue with a gasoline engine.
The most common universal grades are Shell Rotella T, Mobil Delvac and Chevron Delo 400. The most common weight is 15W40 which covers virtually any climate.
A new universal grade, and the only synthetic I know of is: Mobil Delvac One. If I were going to use a synthetic in a diesel, this is the only one I would consider for the purpose.
Shell Rotella and Chevron Delo are available at Wal-mart and go for about $5.50 per gallon. These are readily available at Truck Stops on the Interstates and also your local oil suppliers. If you check in your area, you will probably find a few businesses that sell farm fuel, oil and other products to the service stations and such. You can buy them there by the case, pay the tax and save just a little. The main advantage is that these oils which are not so common at the auto supply store, are commonly used for farm or construction equipment use, and available there.
I have run all my diesels on Chevron Delo 400 with great success. I also use it in all my gas engines from my lawn mower, to my four wheel drive, to my wifes 4Runner. The added detergents keep gas engine crankcases clean enough to eat out of.