Oil viscosity

Yes. Attacking the grain structure was cited in the debate linked early Rebuttal to Shell's Zinc Myth statement. The rebutting side did not dispute that part. (Unfortunately the specific study isn't given, but probably can be looked up in SAE's papers or conferences.)

Widman discussed the balance of ZDDP with other additives (page 13), as well as the reduced need for additives with better base stocks. (pages 10-12). Widman Engine Oil pdf