Synthetic oil viscosities for heavy-duty towing

[ipso_facto]

It is 7.5L ford engine, sometimes used for towing up to 9,000 lbs.

What's the proper viscosity of say Mobil 1, 10w-30? It does consume a bit more oil at higher rpms, as is typical with all 460's even newer ones. The owner's manual calls for 5w-30.

Secondly, differential oil.
I got some synthetic 75w-90, then was told while it would work in F150, for heavy duty stuff and serious towing, I need 75w-140, which is what Ford specified anyway. 75w-90 stuff is good for front differential, if you have 4x4 (I don't). 75w-140 is pretty hard to find in synthetic, might have to order it online.
BTW, the official Party Line is that no gasket should be used. Just RTV (silicon).

Most importantly: Tranny.
It has been serviced a year or two ago, and hasn't towed much since then (based on previous owner) so I sitill have about 15K miles before anything needs to be done with it (even in the severe usage schedule).
But I wonder if I should do a partial fluid replacement. That is, remove the pan, filter and whatever oil comes out of it. Then replace what's lost with synthetic, creating a custom a blend - or is that a bad idea? I don't think it deserves a complete flush just yet. This shade tree mechanic method is supposed to be equivalent to a full flush if done 3-4 times as I understand. What's the a good synthetic tranny fluid and viscosity (auto C6 tranny) and filter.

I am thinking about an aftermarket tranny pan which is just like an oil pan - loosen a bolt and the contents come out. Instead of messing with RTV/16 bolts. Though it doesn't address the filter change of course. C6 is supposed to be pretty reliable, and it is.

[87MH]

As far as the engine, stick with the manufacturer suggestion for viscosity, but 10-30 is certainly a reasonable substitute for 5-30, unless you often see temps below 0 degrees F.

You did not say how many miles your engine has on it. Several user group threads do not suggest switching engine oil to synthetic on a high mileage engine.

If a rebuild is planned on the engine in the future, after a run-in with Dino oil, a switch to synthetic would be appropriate.