Well, in your shoes I would call Hydrogear themselves. They used to have real people in their tech center that were helpful. Possible improvement may be a spec change on the oil or maybe pump parts that you find that you can do yourself if you are up for it. May be there are other service numbers that could easily replace the one you have that would be readily available slash affordable. Strangely enough Google might link you with a wholesaler or Ebay may have something available. It would be interesting to hear what they say.

Don't just cross it over to an oil filter. Transaxle filters usually have a tighter micron rating and different internal construction than oil filters. Google is your friend. Here's the REAL one (not a chinese knock-off) at Amazon... http://www.amazon.com/Hydro-Gear-%C2%AE-FILTER-SPIN-HG-52114/dp/B000PZVE64

Remove filter, oil drain out of filter port. No drain plug per se, but the lowest hose fitting is what Hydro Gear uses to drain the oil Remove the lowest hose and fitting, allow oil to drain. Open the oil fill plug (on top of case), open the oil level plug (on side of case in vicinity of parking brake caliper) Let the oil drain out of filter port and lowest hose port for at least an hour. Install filter, reconnect lowest hose. Pour 20 W 50 motor oil in the fill plug very slowly and watch the oil level port for oil to run out. It could require as much as 98 ounces of oil if the trans had been apart for service, but yours will have some trapped oil sequestered inside the pump and motor, so expect it to hold around a quart or a little more. After filling, run the transmission with the drive wheels off the ground for at least a minute and then recheck the oil level. And that's it.

I grow tomatos outdoor using the EBB&Grow 12 plant system from CAPP. Pretty much a 55 gallon drum and a controller bucket, which flood the buckets at pre-determined itnervals throughout the day (usually 2 or 3 waterings per day). I have had great success with this outdoors, no problems yet in fact. I ma starting on week 5 now and My tomatos and cucumbers are doing great. Ask me if you have any questions. I love ebb&flow style hydro and it's perfect for outdoors. I did have to buy a chiller to keep the water cool enough though because I am in Northern CA and it gets kinda hot here during the summer. Here are some pics: My nearly 5 foot cucumber plant on week 4: http://i.imgur.com/pDjz6.jpg