The rotational speed of the shaft has a significant effect on the desired clearance. As a rule of thumb, I'd suggest that the minimum film thickness for grease or oil would be about .001" and part of this requirement is due to inevitable particle contamination of the lubricated zone. Once some wear particles start to accumulate, they'll quickly lead to seizing of the shaft in the bushing. So that means the bushing should be .0015 to .002" over the shaft size. If you give it even more clearance, it probably does not help, because then the excessive grease or oil simply gets pumped quickly out the end of the bushing because of the loss of capillary action, which is part of what keeps a film intact.