I don't think there is enough varience in performance to worry. Make sure there is enough to keep some preload after it gets beat on. Otherwise it can beat tthe shims out. The most important thing I've learned is to loosen the backlash up to .010"-.012". If running tighter as the teeth flex they stay bent slightly and loose backlash. Then threre isn't roo for lube causing the pinion teeth to crack. Most drg race gear guys I've talked to don't like synthtic fluid either.
Doug