If I remember rightly, you have a 300Tdi. I would definitely go for the best oil possible, especially if you are using it in desert conditions. And that means a synthetic oil, designed for diesels.
Never use a synthetic or semi-synthetic oil during the run-in period of a car. The almost total lack of wear that results will mean that the engine never beds in, and the bores will get polished and will not hold oil properly thereafter. Use a mineral multigrade oil for the first 5000km or so, while driving it very carefully.
Then switch to synthetic, and (if you can) stay with synthetic. (There is no real harm in switching back and forth, imo, but a synthetic makes such a big difference to wear rates that it's worth sticking to it, even at several times the price of lesser oils.
Look for a multigrade with a very wide viscosity range - a 5W-40 or even better, a 0W-40. In hot conditions you would want a very "thick" oil, because it will thin down in the heat. But then you have the problem that it doesn't reach the wearing parts of the engine very fast when you start cold. Hence it is a huge benefit to your engine to have a multigrade with a maximum viscosity rating of 40, but a minimum viscosity as low as possible.
The service category of oils (defined by various standards bodies, ACEA is the major European one) is a complex measure of a whole variety of other qualities of an oil - the ability to withstand chemical break-down due to contaminants, the ability to protect the engine from corrosion, etc.
ACEA B3 is the highest rating for available oils suitable for light duty diesels (as opposed to trucks, earthmoving equipment etc.). Look for this rating on the packaging. B2 is rated not quite as highly.
In most African countries, diesel fuel has a high sulphur content, so you need an oil with a high Total Base Number, which can cope with the acidity caused by the sulphur. Truck oils usually have a high TBN, but are often not good for smaller engines. The ACEA rating for a top quality heavy duty diesel old is E3. (A3 is the top rating for petrol engines). There are new 2002 ratings of B4 and B5 for small diesels, but I don't know of any available oils that comply with these. The same applies to ACEA "A" and "E".
For marketing reasons, oil companies will seldom package the same product for several purposes. Nonetheless, some oils have been tested and carry service category ratings for more than one type of engine. Sometimes these additional ratings are not even displayed on the packaging, though some research will identify them.
Mobil Delvac 1 SHC come very highly recommended. It's about £100 for a 20 litre drum, though! It's a fully synthetic 5W-40 oil, and rated both B3 and E3. Because of the E3 rating (for trucks), it has a very high TBN of 16, which means that in a 300TDi or TD5 it is suitable for use in areas where fuel has a sulphur content as high as 1.6% - pretty much anywhere.
(Incidentally, fuel in Europe is legally restricted to below 0.05% sulphur, but in Morocco it can be as high as 1%!)
For more information on ACEA ratings, see http://www.infineum.com/information/tables.html
Some synthetic oil manufacturers claim that with synthetic oils you can extend the oil change interval. I would agree that it's better to leave a synthetic oil in for too long, than to leave a mineral or semi-syth oil for the same period. But I would definitely stick to the recommended oil change intervals, or even shorten them if you have been travelling in harsh conditions and the vehicle has been working hard.
Furthermore, if you can't get a good synthetic oil at the time, it's probably better to change the oil anyway, and put the best available in instead. The actual decision would probably depend on how bad/unsuitable the available oil really is, and how far you would have to run to find a better oil.
For gearbox, use the correct type of ATF (Dexron III, I think) as specified in the manual. Don't chuck in oil (except in an emergency of course).
The transfer box takes 80W EP oil, as do the diffs. Make sure it all conforms to API GL-5.
To the best of my knowledge the synthetic/semi-synthetic option for Land Rover Defenders therefore only applies to the engine oil.
[This message has been edited by SandyM (edited 17 January 2003).]