Ok I've searched a good bit and the results have been a bit conflicted. So I'll ask the audience.

I'm running right up on 369,000 miles and will soon be swapping out the oil in the diffs and transfer case. I know a lot of guys use the fancy synthetics and that's great, but I don't have the cash to drop on synthetics, plus I don't want anything to start leaking since I'm a "high mileage" guy. I figure if it made it to nearly 400,000 miles without synthetic oil—I'll just go ahead and stick with old fashioned dinosaur juice.

So my main question is, can I run the more common 80-90 conventional in all three (diffs & transfer) without issue? Or is there some reason that I need to stick to 75-90 which is hard to find in conventional.

I know I'm going against the grain here as most people seem to use 75-90 synthetics, but again, I have a long list of maintence things to get through and need to save cash where I safely can.

Thoughts?

Your location is listed as "The land of fruits and nuts...". Whats the weather like there? If you have a mild climate, I'd run the 80-90w. If it gets cold, then I might be more picky and stick with the recommended weight.

I would stick to what got you to 369,000 miles.

California...pretty mild here in the Central Valley. Typically runs from a high of 105-110 in the summer, down to around a nightime low of 30 in the winter.

While I can 100% guarantee that the PO never put synthetic in it, I can't say what viscosity was ever used.

Honestly I'd just get some Valvoline 75w-90 conventional for all three. It can be found at Autozone, AA, Oreilys, Walmart, etc. 80w-90 is pretty prevalent at the locations mentioned as well if you want such for the rear.

Make sure that the fill plug for all three will come out before draining it. Don't forget new crush washers.

Also make sure you rear breather is working well. If in question, replace it.

....

Just a heads up that I replaced the oil in all three of my diffs with synthetic at 180k and within a month the center diff started leaking out of the rear seal. At this point I think I agree with the above recommendation to go with conventional.