I DO NOT recommend anyone use engine oil in a T5. It doesn't have the correct additives for use in a transmission. There are special shear additives that are absent in motor oil. While folks have done it and gotten away with it, it is more because these old hot rods and Fords don't see near the mileage of daily drivers.
I run synthetic ATF in all T5 - WC & NWC. I find the GM synchromesh too thick and the gear oil originally called for in early T5 to be WAY too thick (the old Ford oil translates to a 70W gear oil). It doesn't hurt anything, but it makes them sluggish to shift, especially when cold.
I suspect end play. End play is the easiest adjustment one can do on a T5, although you need to pull the top & tail off to do it. I set mine to the tighter end of the tolerance.
Is your whine less in 4th at all? Generally, 4th has the least noise of any gear, but if it's noticeably & significantly quieter than the other gears, it is indicative of a bad input bearing...although too much end play can be almost as bad. If you have way too much end play, that will result in worn/broken synchros and dogs on the gears.