Gear oil discussion

I have a 2000 4w4 lifted (155k) with 305/70/16 tires, and 2002 2WD (245k) with normal height and tires. I'm researching and preparing a front and rear gear oil change. Manuals states 75w90 on front and 80w90 on rear. I live in Idaho where there's some cold winters, sometimes sub-zero, and very hot summers often >100 deg. I'm a novice at changing these, but with a bunch of vehicles it's time to learn. I've watched and read a lot on this and they are within my ability.

I compiled a list of my 6 vehicles (including 2 Tundras and a 2004 4x4 4Runner) and lubricant needs, and what I have on hand that I'd like to use, if possible. But not necessary. I want the best, right lube for the job.
I have
· 3.5 QT 80w90 GL5 partials conventional
· 0 QTs synthetic 80w90 - need 19-20 QTS for all 6 vehicles, that’s 5 gallons.
(Some recommend 75w110 as a suitable replacement, even 75w140. Not sure.)
· 8 QT 85w140 Chevron Delo
· 8 QT 75w90 syn Valvoline - 3 QTs in Tundras front

Searching around, there's a dizzying array of choices. First would you go with synthetic or conventional at generally 1/3rd the price? Secondly, what good or bad experiences with brands and weights have you had? Also, could I mix some of the various I have on hand, to get a very close approximate lube weight of about ~80W120??

A 5 gallon bucket of conventional STP or Valvoline 80w90 would be enough for all my vehicles and is $80-$100, so very affordable per QT. Looked at Redline and RP and Valvoline and other full synthetics and it's closer to $15 per QT, so my supplies go to $300 for 20 QTs. Autozone has a 5 gallon jug of STP 80w90 for only $73, which looks very appealing... Likewise Walmart's Supertech 80w90 is $15 per gallon, so similar price and 5 separate jugs is easier to work with.

Any tips are greatly appreciated.

Thanks.

 

I mean personally, I wouldn’t put too much thought into it. For gear oil, I’ve always ran Mobil 1 in the appropriate weight. It’s never given me any trouble in any of the vehicles I own. I wouldn’t mix weights and I would opt for synthetic at the very least. Conventional is fine if you change it more frequently.