Adding fuel to the fuel tank is absolutely unrelated to to any change in engine oil pressure.
The oil pressure will be higher when the engine oil is cooler and lower when the engine oil has warmed up.
If you are experiencing abnormally high oil pressure on start up I suspect you are running the incorrect type and grade of oil in the engine. Only use the oil specification(type and grade) listed by the manufacturer for your engine and nothing else.. If you are unsure what that is then it is specified in your owners manual for the vehicle.
High fuel consumption has also nothing to do with your engine oil pressure. If you are experiencing high fuel consumption then please read and examine these factors..........................
Finding and Fixing the Causes of Poor MPG
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