Viscosity has an effect on the original Rhoads lifters. Rhoads say's it doesn't.
I 've ran enough of them over the years and I know for a fact there's a difference in how they work at lower rpms using 20/50w vs 10/30.
You would certainly think viscosity would affect them some. I don't see how it could not. I believe they have a viscosity recommendation for their lifters, so that tells me right there it has some effect.