Interesting that you say your leaking gasket caused an erratic idle.
Its not the oil leak causing that its air getting into the enginen past the leaking gasket, this then messes up your fuel trims as the cars exhaust sensors pick up extra air that has not been metered by the Maf, it fools the car into thinking its running lean so demands more fuel at the injectors to richen the mixture to counteract the lean condition it thinks it has. This will go on until its adding up to 20% more fuel it eventually realises something is wrong and sets the EML and a code in relation to fuel trims. If you have this going on you will certainly notice your mpg drops and you will be visiting the petrol pumps more frequently.
When renewing the cam cover gasket ensure you put new round rubber seals in under the cover retaining bolts, these go hard with age and heat cycles and dont compress well, reusing them runs the risk of your cover not sealing down enough.