My first thought was too heavy oil also but you have used the right stuff.
The other replies are also valid like a suction leak.
You did not mention pressures at all. Main (clutch) pressure should be 160psi-180psi. You should have this in neutral, in any gear, at idle or hi idle. Do you know what the pressures are? Does the machine have a gauge on the dash? If no gauge you can tap in a gauge at a pipe plug on the selector valve on top of the transmission. This is the main pressure port. Use a 300-500psi gauge, not one that goes to 3000 or something.
Another thought has to do with the selector valve. It has three spools, a F&R spool, range spool and a declutch spool which is activated by the left brake if it is still stock. The shift spools are probably ok if the detents feel good but maybe a problem in the declutch spool. Disconnect the line to it, some were air, and try it. If the declutch spool is not moving properly it will not allow pressure to get to the clutches.
Torque converters are rebuildable but probably not your problem.
Fast Shipping! Most of our stocked parts ship within 24 hours (M-Th). Expedited shipping available, just call! Most prices for parts and manuals are below our competitors. Compare our super low shipping rates! We have the parts you need to repair your tractor. We are a company you can trust and have generous return policies. Shop Online Today or call our friendly sales staff toll free (800) 853-2651. [ About Us ]
TRADEMARK DISCLAIMER: Tradenames and Trademarks referred to within Yesterday's Tractor Co. products and within the Yesterday's Tractor Co. websites are the property of their respective trademark holders. None of these trademark holders are affiliated with Yesterday's Tractor Co., our products, or our website nor are we sponsored by them. John Deere and its logos are the registered trademarks of the John Deere Corporation. Agco, Agco Allis, White, Massey Ferguson and their logos are the registered trademarks of AGCO Corporation. Case, Case-IH, Farmall, International Harvester, New Holland and their logos are registered trademarks of CNH Global N.V.