Hi and welcome. my two pennyworth, check the drive on the dynamo is tight. May also be worn or stuck brushes. But you need to check with someone like Richard Farrant, I think there is a funny the way the dynamo is driven off the timing chain. Check all connections are clean, dynamos are touchy that way. A good spray of contact cleaner or WD40 into the back of the Dynamo might cure the problem. Dynamos are quite simple to work on, but you must remeber the re polarise it after work, BEFORE you refit the regulator wires. Oil, well 30 is the right stuff, but if you flush the engine out well 20/50 can be used (Wait for screams from Degsey! It is an on going discussion:D). As for the leack, where is it coming from? As for surface rust, a good clean up and application of a rust killer, I swear by a stuff called Vactan, then re paint. A Military Vehicle is a working tool, so a bit of battered look is in keeping.