So I looked up the transaxle as 4900-3 and it shows a separate pump of part # 5376. Maybe that will help. I seem to remember literature that said 10W 30 or 30W for this lt style pump- but I can't find it and never did anything with them. I take yours doesn't have the overflow cup? Honestly, I can't think it would drive any better w/ 30 than your 20w50. You could be on target on needing more however.

Tomplum, do you have any idea who I might find the replacement pump. I found a site that sells the whole shebang for a mere $1755, but it will be a junker before I go that way. I'd almost bet the pump itself is pricey, too. What I was asking and thinking is this; If I put a heavier oil, such as a lightweight gear lube, would it perform better, i.e. as in the use of heavier oils for a worn engine. I was thinking of draining the oil I just put in and replace it with Pennzoil Synchromesh gear lube like used in manual transmissions. It seems to be about 45w, thin enough for full flow, yet thick enough to allow higher viscosity after heating up. My thought was to increase the pump pressure in the piston area to get near to full function and not have the oil thin out so much and allow cavitation. My unit does have a plastic fill reservoir to fill oil up in and inside the reservoir is a cone shaped rubber cup, which I guess acts as a expansion cup for the oil to have room to expand into the reservoir by compressing the cup upwards in the chamber.Anyone have any thoughts, please chime in, I'm just like a blind boxer, just swinging away and trying to hit on something.

Well, that you were only able to add about 24 oz of new oil sorta tells me that there was a lot of old oil remaining in the transmission assembly, I can't imagine a hydrostatic transmission having a capacity of less than a quart. So I think what you may be experiencing is foaming and aeration continuing to develop after the oil heats up (though it "might" be less than before you tried to dump the old oil). As for switching to a "gear oil", I would not recommend that as the additive packages for the two different oil applications are very different, viscosity is not the only difference between "gear oils" and "hydro oils". I would say you need to do more to get all the old, worn out, "additive depleted", contaminated, oil out and refill with 20-50 motor oil. Try that and if you see a definite improvement in performance, use it a while and repeat the drain and fill again, the second time fill it with synthetic 20-50.
I have included a link that has lots of info for interested parties.

Here is a link that might be useful: BITOG site

Mownie, now I suppose the oil info site was to be helpful, but I didn't know where to begin. I'm not to technical. But I get the gist of what you were saying. I think. But at this point, which is after I spent the day working on this and several other mowers, I did drain the new 20w50 out and let it drain for at least 4-5 hours. There was no drip, drip, when I put the plug back in and filled the reservoir, with full synthetic 75w90 gear lube. It had the viscosity look of regular motor oil. By the time I put it in, I measured 16 oz as opposed to the approximate 24 oz that I put in before. I had spilled some in the first effort. And the rubber "cone" I described before turned out that I hadn't "folded" it up into the lid of the reservoir. It now looks like the rubber inside a brake fluid reservoir lid on a car/truck. This gave me more room to put more in. As to the possibility that I should be draining more than a quart, I don't know how I would do that since I have only one plug to drain from. At least I haven't been able to see another one. The only other way would be to remove the axle and flip it upside down and let it drain oil out through the reservoir opening. I purged the system of air, but it got dark on me so other than testing the gear lube out while it was up on jack stands and it at least it spun the tires both ways as expected, just hope it will pull its weight up a hill, too. It will receive my usual torture test. I will report back after I trial it tomorrow pushing/cutting the leaves off my hilly yard. It either will work or it will be junked out for a loss.

***"remove the axle and flip it upside down and let it drain oil out through the reservoir opening"***
You have hit the nail squarely on the head! Once again, it comes to saving a few dollars at the assembly plant by deleting a drain plug opening and providing a threaded plug to fill the hole. There have been many threads and posts in this forum where folks have had to do this in order to get all the bad out. And look at the options.......if you can't get the tranny back to health by any means, you are going to be removing & replacing the tranny, or junking the tractor. So, you might as well R&R the tranny for a complete upside down drain out. Yeah, the oil guy website is a big read, and you would have to dig for the info about the various types of oil and how they are formulated for a specific application. I guess I can restate that gear oils are formulated to withstand extreme pressures of hard "metal to metal" surface contact that is found in gear type transmissions, drive axles, and final drives. The oils for engines and hydrostatic drives have other problems to deal with, such as the ability to keep from foaming and aerating while at the same time being able to flow easily through the pump and any filter media that the engine or hydro may have. I recommend that you get away from gear oils, they just ain't meant for hydro applications. You may do more harm than any hoped for good.

Not belaboring your point of oil degradation due to foaming in a hydro pump system. And your experience obviously is way ahead of mine, but I have a drain plug on the bottom of the unit. It drained out approximately the same as I put in. One of my original questions was, "How much oil is the capacity of this particular Spicer/Dana/Foote unit? But that being asked, I have done an oil change with very good improvement, using a 20w50. I now have drained that out and I have already put synthetic 75w90 gear lube in it and purged it and tested it on the jack stands. So either I try it as it is set to go now, or I dump out that and retry another motor oil with heavier viscosity. I think you are saying that gear lube will foam while motor oil won't. I was hoping that the synthetic factor of the 75w90 gear lube having a lightweight feel to it (very unlike STP) would actually work to keep up the pump pressure, and not foam and still allow the flow for movement of the oil within the pump. Again, I concede you are the expert here. I'm just the blind boxer trying for a lucky hit.

Well, I don't think I qualify as an expert in hydro trannys, and I clean forgot about your mention of a drain plug. But I maintain that gear oil, synthetic or not, is not what you want in a hydro transmission. Foaming of the oil can be caused in two ways, one way is when moisture becomes entrained in the oil over time by the natural breathing out and in of the tranny air space due to warm up & cool down cycles of operation. When the oil has enough water in the form of an emulsion, this water may "flash" into steam when subjected to the high hydraulic pressures of the pump and the hydro motor. The other way oil can foam is when internal moving parts sling or throw the oil throughout the air space inside the tranny, kind of like when you direct the discharge of a garden hose into a bucket of water,the water foams up but loses its foam as quickly as it forms. Similarly, the oil foams but quickly loses the air if the antifoaming additives are good. When the additives wear out (or aren't there to begin with), the foaming may be persistent, like adding soap to the bucket in the hose scenario.
I wish I could tell you the capacity (oil volume) of this hydro but I don't know that and I've not found it searching the 'net.

Ok, that now makes sense to me. You just have to break it down to the soap in the bucket or K.I.S.S. for me, and sometimes that doesn't even work, it takes a 2x4 over the head! I see your point in the foaming scenario now. Well, I wish I could get some heavier synthetic motor oil and in a straight weight, to boot. But the best I found for synthetic was 10w30, and I just kinda think, or my gut tells me that it needs a 40+ weight viscosity. It just acted like the 20w50 wasn't heavy enough. But it might just be that it's just plain worn out, it is after all, only 13 going on 14 years old. I was hoping to get it going good enough to feel like I could sell it without it crashing on the next person in short order. I'd like to think I rejuvenate a LT to where someone gets at least a whole mowing season out of it. Then they haven't totally wasted their money on a mower.

I understand the frustrations of trying to "raise the dead" so to speak. I admire that you want to turn out a good machine for the next guy. The multi weight oils are actually best for hydro applications (just as they are for engine applications). What we refer as "weight" of oil is actually a "viscosity index". The nickname "weight" came about due to the popular layman's use of "light" to describe thin or very fluid oils, and "heavy" to describe thick or "viscous" oils. Now some 100 odd years later, we sill use "weight" as referring to viscosity. Straight weight, or single weight oils are thicker when cold (more viscous) and thin out when hot (less viscous). Same holds true for multi-weight oils, the "special property" of multi-weight oils is their ability to maintain a specified "viscosity index" throughout a range of temperatures. Some (actually a lot) of folks still think multi-weight oils have some magical ability to "thin out when cold" and "thicken when hot". That's not what the two reference numbers mean. Take 20 W 50 for example, the "20" means that this blend of oil will thicken as its temperature is reduced, at about the same rate as if it were a "single 20 W" oil. The "50" means that as the temperature of the oil increases, it will thin out at about the same rate as a "single 50 W" oil. Therein is the "magic" of multi-weight oils, they are designed to maintain a specific range of "viscosity index" within a specific temperature range. It is easy to see how the story on multi-weight oil gets twisted.
Where have you been searching for your synthetic motor oil?
In middle Tn where I live (Hendersonville) I have had the best luck getting 20 W 50 at Pep Boys auto centers. I found it available in several brand names but I settled on the Royal Purple brand because (at that time) there were several members of this forum who gave it high marks. The Hydro-gear tranny in my old White Outdoor GT 185 called for "99.8 oz" of 20 W 40 motor oil. Several members stated that the recommendation for the oil had changed to 20 W 50, so that's what I bought and put in. I remember that I could only pour about 90 oz or so in and the oil was up to the "full level check hole".

I'm not believing this! I live in Hendersonville, too.
I usually go to Autozone for my oil or the new Walmart whenever I'm going by there. They just don't have as much selection, or least I don't think so. I live near Long Hollow Pike and Center Point Rd. We need to hook up sometime and grab a cup pf coffee!

Yeah, e-mail me.

Well, I ran with the synthetic 75w90 gear lube I had in the trans already to see how it would handle. It handles going up my hilly yard just fine. Unfortunately, when I was pushing a pile of leaves my idler pully broke. I am guessing a stick or something got pushed into the belt and pulley path, and snapped the plastic pulley.