Lemme tell ya a little story about a man named Jed.. Ya get that .. or you don't.. doesn't matter either way.
The "D" series all share the same basic trans.. I had a D-160, and it utilized the "Crap - Torque " K-46 trans. That being said, there's evidence that they are not all the same, despite having the same model designation. The quality of the internals varies in it's different iterations. It's possible to have two, seemingly identical K 46 trans, that have different spec'd internals. Hence the necessity to have the serial number to order "bits".
Since I have a rip roaring case of OCD, when I bought my 2014 John Deere D-160, I read the manual cover to cover. Imagine my surprise / shock, when I came across the following. The basic warantee is 2 years or 120 hours. .. The transmission is sealed for life, and as such needs no service. Should you encounter issues, please refer to your nearest authorized Dealership. WTF? Also, John Deere .. in their infinite wisdom, spec'd the transaxle ( at least on mine), without a drain plug. What did they save per unit? Maybe two bucks? They also filled the transaxle with the cheapest dino oil that they could source. I guess, in their opinion, that the unit would not pooch out until way after the warrantee expired.
I hit 50 hours on the meter, and then ripped the transaxle out of the tractor, popped the vent cover off, and upended it over a bucket overnight. I refilled it with Amsoil Premium Synthetic .. What a difference.. Damn thing would wheelie!! ( well ..OK.. I'd have to give the steering wheel a bit of a tug..LOL).
As others have stated, keeping the fan and fins free of debris is crucial. That can't be ignored. Ever!.
Take the trans, ( if yours has no drain plug), pop the vent cap, and drain it over a bucket overnight. Re-fill as per spec with something premium , and synthetic. At worst you're out $30.00 for oil.. It can't make it any worse. IMHO, the oil should have ben swapped out at 40 to 50 hours, and then again every 200 hours. But, that's just my OCD. The manual, written by the bean counters who now run the firm, wasn't written with longevity, or your best interests in mind. Just because it's green, doesn't mean it's a John Deere. Well, at least not like your Grandpa's John Deere.
To add to longevity, mow across hills, if possible, not up and down. Always run high R.P.M., ( it maximizes cooling over the transaxle ). When you blow off the deck, stick the nozzle up under the seat and hit the transaxle fins too. When you pressure wash the deck, do the same.. That green coating of crap over the fins, acts like an insulating blanket that keeps heat in the transaxle.
Additionally, a modern Premium Synthetic is much more able to operate effectively at higher temperatures, without a loss of viscosity, than the crap John Deere filled it with .