I have two Backyard Buddies and I have never had this happen. I put a Mobil 424 Hydraulic Oil in both of mine and have had no trouble. I have never changed oil in either of my lifts, and I have had them for about 5 years now.
This may be an oil issue. Make sure whatever oil you use is specifically an oil for hydraulic use only. It would be the same type of oil that would be used in a hydraulic excavator (backhoe). I know that Mobil as well as Chevron have good oils that are for commercial use. I could get the most recent Mobil and Chevron brand numbers so you can get one of these, or convert over to another brand. Just let me know if you are interested.
Mark
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.