Best gear oil for a higher mileage 4X4?

[gotek]

Just bought a high mileage Nissan 4 cylinder 4X4.

Everything is OK (as far as known) with the 4WD driveline, except for some slop in the shafts coming out of the transfer case. No leaks of any consequence.

Manual calls for GL-4 in the trans...GL-5 in the transfer case and diffs.

Questions or opinions...

* can I use GL-5 in the trans too...and why not? Can I find GL-4?

* wanted to use Mobil 1 synth...but they only want $9 per qt now....so I might want to use Valvoline semi-synth or regular gear oil...and change it out more often.

* with the dino gear oil...I'd like to use a good additive...prefer moly...but maybe Militec or similar?

Maybe Lubegard's gear oil additive?

* talking about 9 quarts overall

** wanting better mileage and duriability...on and off road...

Any help appreciated.....

 

[unDummy]

I would invest in synthetics everywhere. Redline, Amsoil, Neo, Royalpurple, Specialtyformulations, Motul, have great fluids. You get what you pay for, and IMO, synthetics provide the best durability, reliability, and MPG!

For the X-case and 'ends, I'd grab the Mobil1, Valvoline Synpower, or RoyalPurple Maxgear, which usually are locally available at various autoparts stores. I've also seen the Pennzoil GL5 synthetic occassionally at wallyworld for $8 quart.

For the manual tranny, you can't go wrong with the RL MT90, Ams MTG, or SF MTL-R fluids. Pennzoil also has a GL4 75w90 that is difficult to find(might be in stock at a quick lube, tranny shop, or check yellow pages oil distributor).