A slightly odd one posted by someone on LR4x4 but an enlightening answer nonetheless:

[the additives in the oil are] supplied by either me, or my equivalent in one other company in the world. So you've come to the right bloke. In Shell's case, they take a bit of both. This spec corresponds to our competitor's product, but the advice would be much the same in either case.

Right, first let's do the viscosity grade.

10W30 is definitely thinner than a 15W40. The xxW bit refers to low temperature properties (the lower the better for staying fluid). The second bit (the 30 or 40) refers to viscosity at 100C. These are the codes used by engine oils. Gear oils have a similar sounding naming system, so you'll be familiar with 80W90 gear oils. But these two systems are not actually related to each other, which means that a 10W40 and 80W90 could actually have quite similar viscometric properties.

If you want more details you can find it here;

For engine oils: http://widman.biz/English/Tables/J300.html

For gear oils: http://widman.biz/English/Tables/J306.html

…anyway, it depends on whether a 10W30 would be Ok for a Landy. Check the service manual. I'd imagine newer ones would be fine with this visc grade but older ones might not. Mind you, I think Shell Harvella also comes as a 15W40 or a 10W40 (certainly other STOU marketers offer this), so if it's too thin, he could buy a thicker version.

Then to the STOU type of oil. They are fluids designed to go into every part of a tractor; the engine, the hydraulics, the transmission (whether it's auto or synchronised), oil immersed brakes, final drives, some axles etc etc. So they're pretty handy to have around on a farm instead of lots of different pails of oil. It's also quite handy, as Mr farmer can also use the same oil for his Landy and for Mrs Farmer's Volvo. …or at least they used to be able to. The trouble is that as engine requirements become more demanding and transmissions become more sophisticated, trying to meet all of it with one fluid becomes ever harder. So the level of engine technology by modern standards is fairly basic; that's the API CF-4/SF bit of the letter spaghetti. The C stands for Compression, the S for Spark, and therefore shows the level of Diesel / Gasoline engine performance this fluid meets. You can also get STOUs that go up to API CG-4, if you need one extra level of engine performance. Again, here is where your mate checks his service manual to see what spec level he needs. Newer Landys will demand more I'm sure. This may be fine for more venerable machines.

As you rightly say, these fluids can also be used for other stuff. Most of the spec numbers are essentially saying that they've been checked out for multi-purpose use in tractors and work well (hence mention of John Deere, Massey Ferguson, Ford, etc). More relevant to Landys is the GL-4 capability, i.e. for its use in gear applications. These lubes are fine for wherever GL-4 is required. Are the Landy transfer cases GL-4? It also works in tractor synchronisers, so *may* be OK in Landy manual gearboxes - though the particular friction characteristics required by Landy gearboxes may or may not be compatible with STOUs. Anecdotally, I've heard it works well in Subarus and in Mazerattis! But that's not a technical recommendation. Either way, you're unlikely to do any harm to your gearbox. The worst that would happen is the shift-feel would become a bit rough, but you could change the fluid for something else in that case.

For hypoid gear axles (i.e. where GL-5 is needed), this probably won't be beefy enough for the job. It doesn't meet the requirements of GL-5. There is some evidence to show that in tractors, once they've been fully run-in, that you don't really need a GL-5 and can use this type of fluid in its place. However, I'd be reluctant to do that, particularly in a Landy Off-Roading, where the application is quite severe. So I really wouldn't use it in place of GL-5.

You can also use it for hydraulics (though it will emulsify water, rather than drop it out) and potentially even for power steering, though perhaps it's a bit thick.

One final thing. There are two types of universal oil; STOU and UTTO. The names (“Super Tractor oil Universal” and “Universal Tractor Transmission Oil” - aka THF = Tractor Hydraulic Fluid if you're a 'merkin) don't really tell you anything. But STOUs are designed to go into every part of a tractor including the engine. UTTOs are designed to go into every part of a tractor EXCEPT the engine. So don’t put a UTTO into your engine. It'll be far too low on dispersants and you'll have combustion deposits dropping out very quickly.

On to your last question about what the difference is between these types of fluid, it's down to the type of additive chemistry involved. Engine oils use ZDDP (zinc dithio dialkyl phosphate) as the anti-wear, with dispersants and detergents to keep everything clean, plus a sprinkle of anti-oxidants, anti-foam, corrosion inhibitors to balance it all. ZDDP is good stuff for anti-wear, is fairly mild on copper and steel corrosion and helps keep oxidation in check. STOU is based on very similar technology, balanced for a wider range of applications, with friction modifier added to cope with oil-immersed brakes (as found in tractors).

GL-5 gear oil is designed for high-pressure hypoid gears, so needs excellent extreme pressure agents and anti-wear. This uses sulphur / phosphorous chemistry instead of ZDDP. The S/P chemistry is much more aggressive, which allows it to provide the protection to the gears, but is much poorer in terms of oxidation and can be aggressive to copper, brass, etc. Because it doesn't need to cope with combustion, like an engine oil, there is no need for the high levels of dispersant / detergents.

GL-4 is a much less severe application than GL-5, so could be achieved with either type of chemistry. If you were to buy a GL-4 only lube, it would probably be the same chemistry as the GL-5 type, but with about half as much additive (so it protects the gears, but won’t attack the metals as much).

Specific synchronised gearbox oil could be almost any of the above. It needs to protect the gears, but also have the right friction properties. Different manufacturers each do this in a different way.