As far as I know, This is a win-win for Both the attacker and the defender. The maximum amount of oil you will lose for a certain age is fixed, 6000 for CWA with fully upgraded oil refinery for example, if you don't consider the surplus amount from the oil wells. The attacker gain the museum buff as a BONUS (from the system, like domination bonuses) and the added amount does not subtract from you. This is obviously a win for the attacker.
If the defender had Any loot return buffs activated, they will gain back the % of the total amount of looted resource and not the actual amount. Let's see, 2x 12% from museum which should fairly easy to obtain and 27% from temple, this equates to 51%, in other words, half the lost resources.
For Example,
Attacker bonus 50%
Defender looted resource refund 50%
Available resource to be looted 100 oil
Attacker gain resource =100+50% = 150 oil
Defender gain refund = 150*50% = 75 oil
Defender net resource lost = 100-75 = 25 oil
It can be easily seen that you will only lose a minimal amount of resources and the refunded amount is SAVED securely in the defense tab, until it expires after a total of 25? defences.
Isn't it truly a good one from NEXON?