You can only get EXP from Campaign includes Normal Campaign and Elite Campaign. In order to obtain more EXP, it is the key to complete campaigns effectively.
When you complete a campaign for the first time, you will be rewarded with EXP×3. So remember to consume the oil in new campaigns, thus you may obtain more EXP.
Besides EXP×3, you will be rewarded with Gold×3 for first competition of a campaign. Golds are very useful for upgrading ships and parts. So you have to consume oil properly to get 3 times EXP and Gold
What shall you do with the oil left if you cannot complete a new campaign?
From the picture above, you may find out the EXP varies based on the quality of the stage. So you shall complete stages with higher EXP in both Normal and Elite Campaign.
Your Military Rank will provide bonus effect like more EXP, gold, chances to purchase gold and oil every day. Remember to obtain more merit so that you can get the bonus effect.
The Challenge Feast rewards you with twice EXP in Normal Campaign, and twice EXP, Gold, items in Elite Campaign. So you shall use all oil tanks during the event. When you complete new campaign, you will be rewarded with six times EXP. How amazing!
Those are some tips of upgrading. We wish you enjoy the game.