The amount of Achievement experience you get is based on the level of the quest you are doing, and that value is fixed. The percentage that experience translates to is based on your Achievement level. So if you are achievement level 5 doing a level 20 quest you are going to get the same amount of achievement experience if your adventure level is 30 or 80. There is no benefit to mentor and turn in grey quests for achievements anymore.
The amount of adventure experience you get for doing a grey quest does scale some, so you will see a small improvement in the amount of experience you get for doing a level 20 quest if you mentor to 20 from 80. However the difference is going to be fairly small, and as level 20 quests give out so little experience relative to the amount of experience it takes to complete level 80, it probably isn't worth it in the long run (Or in the short run).
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