SOMEWHERE there IS documentation – will check on insightcentral, cleanmpg, etc. when i have time – that even slight overfills cause significantly worse fuel economy. I am sure that I didn't dream this up.

 

When our dealership was doing the oil changes, and routinely dumping in 4.0 liters, the dipstick would show 1/4" to 3/8" over the full mark, that's facing the back of a garage with a slab that's canting slight down at the entrance. Dead level check would likely reveal it even a bit higher.

After one 4.0 liter (dealership) oil change I finally woke up: jacked up the front end myself, loosened the drain bolt, then gently allowed 0.8 liter to seep out. The metric spec. for oil change with filter change is 3.2 liter. I have a glass pint jar (held pickles or jam or something) with both metric and imperial gradations, accurate enough for this sort of thing. I re-torgue the drain bolt, lowered it back down and checked: oil level was spot-on the dipstick top mark.

Anyway, for the next few days I tried to pay attention to the car's behavior. My take: it seemed a little more willing to roll-on, kind of like putting on lower rolling resistance tires. Basically a lower oil level means the cranshaft has to do less sloshing through the oil at the bottom of the crank case.

For what it's worth: the sweet-spot is not even at the full line. The ideal is mid-way between the two marks, like any tolerance. You have a range, somewhere comfortably between the two extremes is the place to be. That said, one practicality: any engine tends to consume a little oil between changes. Considering this, I would move that initial sweet-spot to the 3/4 mark, to give a little cushion. Really: anywhere between half and full is good.

Finally, it's easily appreciated: the volumes we're discussing are pretty small. Anywhere up to a quart overfill, spread over the surface area of the crankshaft, doesn't the raise the oil level a lot. But the manufacturer designed the engine for a range of oil levels, why not exercise some care in staying within that range?

 

To be fair to the dealership:

I just did an oil & filter change (maintenance minder came on with wrench symbol), the first I've done with US quarts (Penzoil Platinum 0W20). I put in 3 quarts, then an extra 8 oz (1 cup). After running the engine a minute, shutting down, letting it settle, and then checking the oil level, it was slightly closer to the bottom mark than top. I added another 4 oz and re-checked, it's now at around 2/3 the way up from the bottom mark to top mark. Another 4~8 oz would put it at the top mark, but I left it as-is, would rather have it a little lower than top mark.

Bottom line, if they let the fluids drain very thoroughly, I don't think you would be much overfilled with 4 US quarts.

 

Data since my first synthetic oil (over)fill: My fuel economy plummeted to a worst-ever 37.0 MPG, based on 470 miles of mostly-freeway driving. My lifetime average is over 42 MPG, so my MPGs significantly dropped following the oil change. Today I went back to the dealer and they drained oil from the drain plug. Now it is at the mid-point of the hi/low dipstick marks, instead of at the high-point. I'll give another report soon – so far, based on the 10 hilly miles from the dealership to my house, the results seem better, and encouraging.

 

Took my HCH back to the Honda dealer. They drained the excess oil, and gave me a free quart of Honda-brand synthetic 0W-20 oil as a souvenir. Result: First tank since adjusted, based on 322 miles, returned 45.62 MPG – nearly nine MPG better compared with the MPG achieved with similar driving when overfilled. Lesson learned.