Not meaning to advertise for them, but I don't mind mentioning a good service.
At their site you can request a free kit, which has a little form you fill out about your engine and a bottle for the oil, that all goes in a bigger bottle.
I found out about them from thedieselstop.com and explorerforum.com (diesel Fords and Explorers). Those dudes mostly get the tests to see how their extended oil changes are doing.
Since I mentioned thedieselstop.com, I got a little discount, and it was $18. I elected to also have the "TBN" test, an extra $10, which will tell you how much additives are left in your oil (for the extended change guys).
They don't need much oil. 3-4 ounces.
Their lab is in Fort Wayne, IN.
Again, I mostly did it since I wanted to know how the LN3'* internals were doing, and hey, it'* doing just fine