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If transmission fluid is leaking into the bellhousing then the most obvious site is from the lip seal between the (stationary) oil pump housing and the (rotating) oil pump drive tube on the torque converter.
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This seal is part of the transmission overhaul kit and so should have been replaced when the transmission was re-manufactured, and the pump drive tube should also have been reground when the torque converter was re-manufactured.
It’s possible to replace the seal without disassembling the transmission but the transmission would have to come out to get at it (approx. 5 hours of work).
The issue with the fluid relates to the different friction modifier packs used in different fluids. These not only determine the base friction coefficient levels but, more importantly, also the characteristic between static & dynamic coefficients for the clutches – particularly the torque converter lock-up clutch (which is designed to operate at a 3% slip level to eliminate torsional vibration). The 5HP24 transmission was developed around Esso (now Mobil) ATF LT71141 which provides a friction characteristic within the boundaries defined in the graph below
If a different friction modifier pack is used in the formulation of the fluid (like in Dexron for example) then the adaptions in the controller will be able to compensate to some extent for the different slip times (& modify the control pressures accordingly) but will eventually lead to clutch failure through inappropriate slip/energy levels.
For that reason, Mobil ATF LT71141 (a.k.a. ZF LifeGuardFluid 5) is the only fluid which should be used in the ZF 5HP24 transmission.
Just my opinion, obviously 
Phil
Admin note: this post has had its images recovered from a money grabbing photo hosting site and reinstated 
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14th May 2012 1:07pm
