johndeere_2210,
I still haven't had time to do the transmission oil yet, only the engine oil (Mobil 1 10w-30). I've already bought the JD transmission oil, so I will likely not go synthetic in the transmission. Frankly, I didn't think too much regarding switching to synthetic in the transmission, but you have me wondering now.
I'd be interested in people's thoughts of also using synthetic in transmission versus only the engine. E.g. Does one or the other provide more bang for the buck? Also, are there any concerns regarding mixing of synthetic with remnants of the original still remaining in the FEL hydraulics? I'm assuming it's a fairly small amount relative to the almost 4 gallons in the tractor.