FWIW, I use Valvoline full synthetic gear oil. I can't prove that it works any better than any other 90 wt, but I think the friction modifiers should help reduce steering effort a bit. At any rate, when I switched to it in the 3A (along with rebuilding the steering box and eliminating the silentblocs), it suddenly became the sweetest steering car I've ever driven (without power steering of course).
The 3 still isn't quite as nice, but I'm working on it. Replacing the felt bushing under the steering wheel with Delrin helped it a lot. The felt replacement I stuffed in there the first time was too thick and didn't wear as much as I hoped.