What oil to run in my ZF gearbox?

I've spent hours reading everything I can find about what oil is best for the ZF gearbox.

I read the GL5 oils are bad for the "yellow metals" in the transaxle, and I've been told there are no "yellow" metals in the ZF, unless rebuilt with cheap parts.

The box was rebuilt by a reputable builder, I know this, with an extra tall 5th gear installed so freeway cruising is not buzzing the engine. I forget the name of the company but it's 3 letters like RBT or something.

I want to run the best oil I can get for my new ZF box.

I've been told "any good 90 weight is fine", I've read about people running the Castrol Hypoy C GL5 that one of the reputable builders here recomends. I found that but it was like $33 a quart (maybe a misprint?).

I've read about people using all the different Red Line offerings, "Shock Proof Medium", "Shock Proof Heavy", and Red Line "MTL".

I found Brad Penn who makes an 80-90 GL4 non-synthetic, that is probably first in the running, my local shop can get it in a day or 2.

I know I might be opening a can of worms here, because the more I read, the more different answers I get.

I'm OK with switching to synthetic, but I've read if you use too slippery of an oil, the synchros don't grab enough, and don't slow the gears down between shifts and cause shifting issues.

SO, my first inclination is to run a non-synthetic GL4 formula oil, probably the Brad Penn (I know Brad Penn is good stuff).

If the GL5 thing doesn't matter, I'd love to run Swepco oil in my gearbox, but it's GL5, so I'm hesitant until I get solid advice or information/proof that running GL5 has no negative effects. I know Swepco is REALLY good stuff, from my Porshe racing days.

I've asked 10 different people and gotten 10 different answers, so I know if I run Brad Penn 80-90 GL4 non-synthetic, I should be fine, but I'd like to hear what others have to say.

My ZF gearbox is very dear to my heart now, and I want to run the best oil I can run in it.

IF, there's no issue with GL5, is there any advantage to running the Red Line Shock Proof heavy or medium? The "Shock Proof" part sounds like it might be a good thing to run, if that's what it does. I don't plan on beating on this box very hard, so again, probably any 90 weight would be fine, but I'd like to hear what others think about the subject.

Thanks for any help or advice,

Mike