replacing the intake manifold gaskets on 2000 Chevrolet Suburban 1500

I am getting 2 Lean codes P0171 and P0174. My mechanic says that my intake manifold gaskets are leaking. After replacing them he says that I have to replace the drain and replace the oil. I thought that it was antifreeze that ran through the Manifold not oil. Oil in the valve covers. Thank you

Asked by for the 2000 Chevrolet Suburban 1500
My 2000 GMC was throwing the same codes I found it was the fuel pump. My truck would run great most of the time I couldn't believe it was the problem. shop wanted 500 bucks i bought one on e-Bay & installed it myself problem gone cost 64 bucks
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He probably just advising you that the oil needs changing as debris from the old intake manifold gasket has possibly gotten into the oil