If there are 5-10 different types of ZDDP and AW and oxidation properties vary between types, how can you tell that a higher ZDDP content actualy gives better AW protection.

Not exactly sure what type of answer your looking for but here goes. If you are testing a lubricating fluid and want to test the effect of different amounts of ZDDP you would add different levels to your mix and test each batch. Analyize each used batch and find which one has the least amount of wear metals present after use.