99 Malibu Problem,Help

Hi All, I had the manifold gasket replaced two weeks ago. brought the car home and it seemed a little sluggish. drove it last sunday on the highway and as i was accelerating
the car hit a flat spot in the rpm and jumped up a 1000 rpm as i was speeding up to enter the highway.i let my foot of the gas and then continued to drive normally.
Later that night night the check engine light comes and a rough idle is present,Drop it to the dealer on monday morning. they say fuel is burning to rich from the engine code and have to keep it another day as the didnt look at it until next to closing.
Tuesday rolls around..same deal...didnt look at it until before close and keep it another day.
Wednesday afternoon is here.i give them a call to see whats up with the car...as i didnt get a call. service says the Mech hasnt entered any info at 2:45 pm yet...get a call an hour later...service want to know if i will cover a oil change. i say i just had on two weeks ago when you did the manifold. he says there is fuel in the oil but doesnt know why its there and they will have to keep the car another day.
My question to you guys is, what are some ways that fuel may get into the oil and could it have been cause when they did the manifold repair?
Thanks for the long read and any help will be appreciated.........John
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