IMHO some fuel ingress into the oil is normal and unavoidable, for example
during the engine cold startup and warmup phase. But when the oil approaches / reaches
the operating temperature, most of the fuel ingress will evaporate (and through the
crankcase ventilation will be sucked into the cylinders), but leaves some petrol smell behind.
As SteffenJ mentioned the hot viscosity of the old oil can affect the oil pressure at idle.
To rule out the warning light sender and the oil pump wear, check the oil pressure with a
workshop gauge, the pressure should be at least 1 bar (14.5 psi) with hot oil @ 900 RPM.
Under 900 RPM the delivery curve of the oil pump drops rapidly (the idle RPM should be checked
with an external tachometer, because the one in the dash is not too accurate). In some cases
the manual idle speed adjuster is not allows to set the speed to the recommended value @ hot