Ok, I get lots of questions about this. I have detailed the link below to explain things for the real technical amongst you. Don't worry, its in english. I've just used the logo for the link, the logo is the Russian national oil company.
But generally speaking don't go for anything to fancy. Remember, these are seventies engines. If you use a full synthetic oil, you engine will use oil quicker than Yeltsin finishing off a bottle of Vodka. Use a normal known brand of oil, but one with a good API rating. Read the link to explain, basically if you truck will be not used for a while, it needs oil with a good API rating.