Yeah it is pretty much impossible to completely eliminate that noise - called "gear rollover", but heavier oil, more oil, and higher idle all help.
I put 15W50 Mobil1 synthetic in mine, and I overfilled it - I put 2 gallons in it. Just leave the fill plug in the side and unbolt the shifter assembly, then pour your oil in there in order to be able to overfill it like I did.
If you understand what the real source of the noise is, you'll see why this helps and isn't just "covering up" the noise...
As each cylinder fires the engine produces a large (magnitude) spike of torque for a very short period of time (duration). At low RPMS the amount of time between the spikes is long enough for the gears to have time to move from one end limit of their gear lash to the other. It is this uneven spiking that causes the gear rattle.What it is is the gears rocking back and forth inside from the forward limit of their lash to the rearward limit of their lash.
Heavier oil doesn't just "cover up" the noise - it is thicker which provides more resistance to movement of the gears and slows the movement from one end of each gear's lash to the other end. They move more slowly from one limit to the other, so they don't slam back & forth between their travel limits and rattle. Putting more oil in it means that the gears are staying submerged in oil - to maximize the damping action, rather than just being coated with oil splashed up from the bottom of the tranny.
I chose the 15W50 because at colder temps it will have about the same viscosity as 15 weight - which is about the same thickness as the ATF that originally comes in the ZF-5. BUT when hot the oil has the viscosity of 50 weight - which is significantly thicker than ATF (or 15 weight) would be when at the same temp (hot). Of course hot 50 weight oil may still be thinner than cold 15 weight - though it won't be nearly as thin as hot 15 weight would. That is the beauty of multi-vis oil. Bottom line, with 15W50 in it mine is not hard to shift when the temp is below freezing OR when it's over 100 outside.
Last thing about the oil - the plate on the side says to use synthetic ATF, so I chose Mobil 1 synthetic because 1) it is synthetic like Ford recommends, 2) it was the only 15W50 I could find at the time, and 3) being synthetic it will last a LOT longer than dino juice without breaking down - getting dark, and burned, and loosing its lubricity - even when exposed to heat & pressure.
One other thing you can do to decrease the gear rollove noise, is to increase your timing a bit. I upped mine from the stock 625 +/- 50 RPMs to 750/800 RPMs. This shortens the time period between cylinders firing making the torque spikes closer together. If they aren't far enough apart for the gear to move from one end of it's lash range to the other end then the rattling noise decreases...
