I'm just reaching here, but isn't the oil change interval for HST tractors much more frequent than for clutch/manual transmissions? The clutch packs and hydraulic motor in an HST will be a lot harder on the hydraulic oil than will a regular clutched transmission. My guess would be that oil designed to last 800 hours in a standard transmission would need changing at least twice as often in an HST. I think can see why 400 hours on an HST. HOWEVER, if it's PTO hours, i.e. a chipper or other stationary PTO device like a generator, I might be inclined to push out the change if it isn't discolored and smelly. Until the tractor's out of warranty, though, I think I'll just follow the guide to avoid any warranty claim issues.
As for the change at 50H, I learned the hard way to make sure to have the right filter ready to put on, or more correctly, that it's actually easier to drain the oil than it is to fight against gravity. I won't be making that mistake again. I lost just over a gallon out of 4+. What came out didn't look as bad as what's in that pan, and since I was adding the loss back to it, plus a little more when I topped off, I had no real problem putting what I captured (in a new clean catch pan) back in the tractor. I used a double layer of muslin in my funnel to pour through, because I didn't want to risk any possible plastic shavings from the new catch pan going into the transmission. I checked the drained filters for suggested copious amounts of silicone sealer and other mentioned objectionable debris, and saw none. So apparently the assembly was pretty meticulous at the factory. The first 50 hours on my LX were not kind hours. I thrashed it pretty hard, but tried to keep the engine break-in limits in mind. Seems I remember reading "Stay below 2400 RPM until after the first oil change." I'm not sure I see much difference in 2400 and 2800 RPM, but I'm pretty good at following rules whether I agree with 'em or not. Even the engine oil I drained from it was barely discolored at 50H. But, Kubota said change it. So I did.
I'm also in agreement that anything that costs $22/gal should last longer, but on the other hand, look how much engine oil costs. A 5 quart jug of Castrol GTX (Edge) 20W-50 is nearly $50 now. It's all a matter of perspective, isn't it? Do I run cheap oil and change more often, or good oil so I get better protection? That's the questions we have to keep in mind. I'm not recommending any oil, but the mentioned oil is what I've run in my old 1992 5.0L for 171,000 miles, and a hot-rodded air-cooled 1800 cc VW that had 400 degree head temps that would kill any other oil I put in it. Hard to argue with success.
