So, I got a good deal on a little utility trailer for our riding lawnmower. Then discovered that towing even that bit of extra weight (trailer empty or lightly loaded) up short hills makes our 19-year-old Hydrostatic transaxle yell things that it usually whispers . . . and after a while barely tries to move no matter the input given. I put my hand on it and it is easily hot enough to blister even my tough manly leathery skin.

Backstory: This riding lawnmower is a 1999 19.5HP Craftsman 42"-cut with a hydrostatic transaxle that we picked up for a good deal about a year ago. Since it will only be used as a lawnmower I decided that a hydrostatic would not be overstressed and would be easier for the whole family to operate. The transaxle is (supposedly) sealed for life maintenance-free yada yada.

Now it got super hot and makes loud noises and barely moves. 20 minutes of idling with a bucket of bird seed on the seat so I can leave the clutch engaged so the little plastic cooling fan will try to cool it, no perceptible change in temperature.

Stuck the hose with a sprayer locked at a fine mist in one of the frame holes so it is spraying over the top of it "helping" the fan. The water coming off the bottom is shower warm. 20 minutes of that and it is mostly cool and will move while growling loudly in protest.

After some research I discover that it takes 20W50 engine oil. Go figure. NOTE: don't take my word for it across all hydrostatics, different makes/models take different stuff. Start another thread to partake in the fruits of my research.

So I go out looking for 20W50 engine oil. I've seen it on the shelves for years in various brands so should be no problem. As it turns out there is no 20W50 left on most shelves anywhere. What little is there is labeled "high zinc" and "motorcycle" and is around $9.00 per quart. Since I'm not sure what is different between "normal" 20W50 and the high-zinc "motorcycle" 20W50 I stay away from it.

As an aside: Castrol has conventional 20W50 in the 5-quart bottles, however, I'm not a fan of Castrol and I want synthetic so I skipped on that. Theoretically, the oil I'll be putting in this (sealed-for-life) transaxle is a commitment I'll be making until the end of time so I want to get it right. The curators of the museum in the space station orbiting Alpha Centauri will appreciate my tenacity in the year 7,000 when they put my lawnmower on display.

I notice that Mobil1 has a 15W50 full synthetic 5-quart bottle most places. After exhausting other alternatives I finally settle on that.

Remove and drain the transaxle. Blegh! . . . and scary seeing that metallic sheen to it. Probably to be expected after a couple of decades of service but still puts my spidey-sense on edge.

Fill it up, drive it around. It recites its protests quietly while cool, and gets quieter as it warms up. It gets good and warm, but not untouchably hot. It is never as quiet as it used to be, however seems functionally happier than it has ever been. I'm thinking in a month or so I'll replace with straight 50W or maybe put 20% straight 50W in with 15W50. I'll likely never mow the yard in winter so I'm not so concerned with the 15W.

So aside from wanting to liven up the lounge as well as to help all mankind with my discoveries, I've come out of this learning a few things and have a few questions for folks that know more about this stuff:

- I bet the designers of this thing assume that (on average) only about five horsepower will ever be used for motivation with occasional surges for hills and directional changes and etc. The other up-to-14.5HP would be used to power the mower deck. Does anyone have a resource available to tell more about how much power the transaxle can handle?

- A hydrostatic transaxle is just a hydraulic pump pumping towards a hydraulic motor, with driver input changing the depth of the pump and motor pistons for infinite range forward and backward. It seems then, that thicker oil would protect the moving parts better and possibly not bleed past pistons etc. as much, with the only sacrifice being slight frictional and pumping losses with the thicker oil. Does this sound about right?

- There is no external means to cool this thing that I can tell (besides the tiny fan on top). Anyone out there have any clever ideas for cooling one of these?

- Any other tips/tricks/etc. that folks might have to improve performance and/or longevity of a hydrostatic?

- And lastly: Anybody know what happened to 20W50 engine oil? It was on the shelves for years then suddenly it mostly disappeared. I'd love to blame Ford, with their (viscosity of water) 0W20 oil in all manner of light- and heavy-duty gasoline engines for over a decade, but even then all of the options (besides Castrol) essentially disappeared off shelves everywhere over the last few months. Seems like an industry decision of some sort.

Market probably isn't there for it much any more.

I would check WalMart if you have one locally. Sometimes the Supertech brand can be handy for some things.

Thanks rjolly87, that'* what I was figuring too, I just wondered if anyone had further industry insight. In the end, it surprised me that Walmart kept straight 30W and 50W on the shelves as long as they did. I guess 20W50 went the same way.

I checked five Walmarts in my area and none had 20W50 anything (including Supertech) except Castrol conventional oil and the various high zinc motorcycle versions in single quart bottles. FWIW I also went to all manner of auto-parts stores, including the ones that have "experts" yell "WHAT CAN I HELP YOU FIND?" as soon as you start past the door jamb.

With this being a forever fill I wanted to go with full synthetic and a high-quality manufacturer. I figure since a hydrostatic is primarily a hydraulic pump, they planned around predictable viscosity. That'* why I settled on the Mobil1 15W50 . The more I think about it, I think I'll add some amount of 50W to it when I change it again in a couple of weeks. It never operates at 212F, well except that one time. So, it is probably expecting whatever viscosity that 20W50 is when it is 100-120 degrees.

Good thing I can experiment inexpensively I guess.