Horses was my first thought too, but I went with wood because I thought it would be cooler.
Keeping oil trade routes open is a good idea too, but of course the problem is that here is who makes oil in 1940:
1. United States - 183 million barrels
2. Soviet Union - 30 million barrels
3. Venezuela - 27 million barrels
4. Iran - 10 million barrels
5. Dutch East Indies - 8 million barrels
6. Mexico - 7 million barrels
7. Romania - 6 million barrels
I don't see a lot of friends of Germany on that list. And it goes down fast after that.
http://digital.library.northwestern.edu/league/le0280ah.pdf, p. 6.
Hence why I'm increasingly wondering if this notion of tripling German vehicle production is actually a realistic proposition.