Since the manual tranny uses straight weight synthetic engine oil, and it's in a hermetically sealed environment, and all it does is lubricate gears, you should be able to go quite some time before changes. Unless of course you "grind" your gears on a regular basis. Then you may want to change it more often.

I believe the automatic trans uses automatic trans fluid, and I would buy it only at a Chrysler or Mercedes dealer. (Their specific brand name & part #).

A while back, Chrysler was having some problems with their overdrive trannys, and I heard they were blaming it on people not using the correct fluid, and not changing it enough. I'm sure "Feets" probably knows the whole story on that.