I tried Mobil 1 motor oil for a while. It's pricey and I couldn't let the change interval stretch out enough to justify it.
I'm getting excellent results with Delo at 4500 intervals and it's 8-9 bucks a gallon. See
here and
here for my Blackstone reports.
With the diffs I change out once a year. Mobil 1 is to pricey for that. I use the Valvoline Durablend over the cheapy stuff though.
I use Mobil 1 syn in the transfer because it only has 1-1/2 qts and see's harsh duty. Only the best there and I change it once a year.
The tranny gets a drain and refill (6qts) once a year with Mobil 1 syn ATF.
Yes, I'm loyal to each of these fluids in each application. When I first got the rig, I went nuts with Mobil 1 everywhere. Thats expensive. It's the best out there and so I continue to use it in the tranny and transfer because they get used hard. There's a ton of good choices for motor oil and I prefer to change it more often so I went back to dino for the cost savings. With the diffs I change once year since you never know when you might get water contamination from deep crossings. My breathers are extended, but I've heard it can still get in. Hasn't happened to me, but I still change it anyway.