Fuel Efficiency for a Gasoline Engine

I have recently carried out an engine oil change, and the
new oil was the same as the old one being used. I
observed a severe drop in efficiency, something which
cannot be attritubed to driving style (it's too much). So my
question is whether this is normal after carrying out an oil
change?

Furthermore, I try to increase the efficiency by trying to
slow down naturally rather than braking, and I put the
gearing in neutral while slowing down. Is this what is
causing the drop in efficiency? , as this is the only major
change in the driving style.