The service/owners manual is always the first/best source. Though 1985 vintage oil specs are very obsolete, they can give you the basic viscosity requirements for a starting point. Any 15W40 API SM/CJ4 is suitable for the engine. Because the more recently developed oils that did not exist at the time the original owners manual was written, you also have the opportunity to avail yourself of the 5W40 oils, which offer much improved cold/cool weather performance. Ditto for the Shell Rotella T5 10W40 SM/CJ4. In certain climates and operational conditions, you can use a 10W30 HD oil (rated SM/CJ4). Bottom line is that any name brand of 15W40, conventional or syn, is suitable.
Gear oil? You got the base 1985 certs already. The transmission spec'ed a GL4 80W90 (low EP additives to reduce bright metal corrosion) and the axles a GL5, which has more then 6.5% EP (Extreme Pressure additives, sulphur and phosphorus) Those are somewhat outdated distinctions these days because most GL5 gear oils (since a '95 reformulation) are suitable for use in transmissions (and say so on the bottle) where a GL4 is spec'ed. You can do one better. There are now manual transmission oils that are especially made for manual gearboxes and some have the right viscosity to work in place of an 80W90 GL480W90 and the rear axle. I'll let you read the labels on those. Some are designed to replace ATF where spec'ed in manual gearboxes and some will replace the 90 grade oils. The T19 in an old Ford is not a hard trans to make happy oilwise and you can pretty much use any 90 grade GL5 gear oil (as long as the label backs that up by saying it's suitable to replace GL4). Some of the later manuals have very complex oil requirements, hence the fancy MT oils. The rear axle was spec'ed for a 80W90 or 85W140 (depending on the application and axle) but again you can avail yourself of modern technology. A commonly available 75W140 synthetic covers you very nicely for all weather and load conditions and will offer a reduction in rolling resistance (read uptick in fuel economy) over an 85W140 conventional oil. If your truck only sees moderate loads, a 75W90 syn will offer even more significant reductions in rolling resistance.
Coolant is easy. The green stuff and add an SCA (Supplemental Cooling Additive). Most of use use Fleetcharge (found at Tractor Supply and other places) which has the proper SCA.
I have no idea what a Doug Nash uses. US Gear bought them, so that would be a good first place to start.
