Semi Synthetic is fine ... but IMHO, 100% Synthetic is even better for an Air/Oil cooled bike like your DR. But as mentioned, synthetic may not be available everywhere in Africa, and as I mention below, if you use Synthetic, make sure its for bikes or that it has no friction modifiers (Use the Mobil One "Extended" or "High Mileage" that I've been using for years.)

Changing oil on the DR650 at 1000 miles is just wasteful, unless its a really crap oil that is the wrong viscosity. A decent mineral oil will do fine going 3000 miles. The DR is very gently on oil for an Air/Oil cooled bike. Synthetic or Semi-Synthetic oil can easily go 4000 miles, IMHO. (I've owned a few DR's, current one has 33,000 miles with lots of hot weather riding .... year round) If you can't get the Mobil One (doubtful in the UK) then get something like Motul Synthetic to start. This will get you 4000 miles into your ride before you have to change. Try to time your change some place that has a good selection of oils. Get the best you can, mineral or synthetic.

If fuel is leaking past your carb float needle, then you need to deal with that. Replace it, and replace that little O ring that secures the float. Always turn off fuel when you stop.
Just in case. A leaking carb can fill the air box with fuel (DANGEROUS) which can then migrate into cylinder, thinning out oil (BAD) or even causing hydraulic lock. This is very rare on the DR's Mikuni BST 40 carb, but can happen on any carb'd bike.

Clean the crud and corrosion out around the bowl, all jets, replace float needle (has a little rubber tip), O rings. After that, should be OK for another 50,000 miles .... if you use fuel filters. Be sure to check the little filter BUILT into the carb on the DR. It sits just inside the intake fuel pipe. Its tiny and delicate. Clean it, or replace it.

Generally true for synthetic oil. Some versions of Mobil One Synthetic contain NO friction or viscosity modifiers. DO NOT use synthetic oil in a wet clutch bike that claims "Energy Efficient" or "High fuel economy". These synthetics have friction modifiers. I'm not sure with other brands of car oils but I have used the Mobil One synthetics I named with good results on four bikes in the last four or five years. Over 150,000 miles (240K kms) combined miles. No clutch slip on any of these bikes, including four stroke race bike.