Oil viscosity - more difference than I would have thought

[bobc455]

So my wife has been trying to maximize her MPG in her driving. She has a '98 Park Avenue (non-SC) with the 3.8.

She pays a lot of attention to the MPG readout. She's been proud to work it up to 27.6 MPG - not bad for such a big car.

At the last oil change, I decided to use up some old 10W 40 oil instead of the usual 10W30. Almost immediately afterwards, her MPG started dropping, and settled out around 26.6MPG (even with her attempts to improve her driving). She asked me what was wrong.

As soon as I realized that the change started to occur around the time I changed her oil, I did some quick calculations and realized that it would be worthwhile (even with the cost of synthetic) to re-change her oil back to regular viscosity. I drained her oil and filled it with 5W30.

As soon as I changed her oil, the MPG indicator started rising again almost immediately.

This past week we drove out to Michigan (about 2400 miles round trip) which was all highway, so the MPG is sitting at 30.5 right now, but we'll see where it settles out when she resumes "normal" driving.

But a 1 MPG difference is MUCH more than I thought it would be, based only on the oil viscosity! I wonder if 5W20 would get her another MPG...

-Bob C.