I agree with EtherealSeraph, you should save up on Stock Fuel for Forced Sorties and/or Mercenary Contracts. 

Also, you shouldn't spent too much credits on upgrades, parts, or aircraft you probably would not use often/ever. It will not make sense to just spend money on upgrading a MiG-21bis, where it could be better saved up for the more expensive ones, such as the FB-22 or the T-50 PAK FA.

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