You will get into all sorts of "debates" around here where lubes are concerned - and any of us are certainly free to use a wide variety of stuff in our engines and drivetrains, if warranty is not an issue - shucks, if ya don't use yer vehicle hard, ya MIGHT even get away with using a few bottles of Wesson oil...
BUT, *ENGINE OIL* in a heavy-duty transmission? Here's what Redline sez about engine and automatic transmission fluids in heavy duty manual trannies, and the NV-5600 in particular:
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These lubricants are not well suited for transmission use because of two factors - shear stability and improper coefficient of friction - which will be discussed in more detail later. At even lower temperatures such as -17. 7°C (0°F), the differences between conventional lubricants and Red Line MTL are even more dramatic. At such a temperature many of these lubricants would prevent shifting into certain gears, depending on the design of the transmission.
All oils are slippery, and with most lubricated components, the slipperier the better, but this is not so with manual transmissions. The synchronization of shifting gears requires friction to transfer energy from the synchronizer, which is locked to the input shaft, to its mating surface attached to the gear to be locked in as the drive gear. Few modern transmissions use sliding gears to change gearing other than for reverse gearing. Synchromesh transmissions have the gear pairs constantly in mesh. The drive gear is selected by using the shift forks to slide a synchronizer ring, which rotates at the same speed as the input shaft, in contact with the selected drive gear. Once the drive gear is brought to the same speed as the input shaft, the locking ring on the synchro assembly is allowed to slide over and lock into the drive gear.
The time this process takes depends on how easily the synchro ring moves and the rate of frictional energy transfer between the two synchronizer surfaces. Higher viscosity lubricants slow the sliding of the synchro ring on the input shaft and require a longer time for the oil to be squeezed out from between the mating synchronizer surfaces. After the lubricant is squeezed out, the coefficient of friction of the lubricant determines the rate of frictional energy transfer between the two surfaces. Slippery lubricants such as hypoid gear oils and ATFs can take too long to synchronize the gears, which promotes synchronizer wear. Red Line MTL and MT-90 has a coefficient of friction which is greater than conventional oils, allowing a quicker transfer of frictional energy.
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SOOooo - get out those cans of motor oil - or Wesson oil, and have at it!
Attention: TDR Forum Junkies 
Could you post some information on this like Gary did from Redline? Seem to be getting conflicting information 
Without some engineering data with testing backup, it would be risky to use anything but what the manufacture recommends. I know some people are in the process of field testing other lubes. If the people that are field testing, would post what they are using with the number of miles they have been using said product, that would also be helpful. By the way, does anyone know what the service life is for the NV 5600, using the recommended lube? How long has anyone gone without changing the lube in their 5600? That might be a good question for a thread 
