What do you consider to be overfilled oil?

Anything between the bumps is fine. Your change in MPG is a coincidence, caused by something other than the oil.

 

When it's running out of the PCV breather tube on the top of the engine. Quick story, about 3 months ago I had a job on a smaller Toyota forklift, it was hauled in to the shop at work and dumped on the steam pad out back, report was it wouldn't run and smoking. So I go out and start checking things out and notice a couple of things, there was oil on the outside of the engine and when I pulled the dipstick out oil ran out of the tube. OK slightly over full, go back and get my drain pan and drain 2 gallons of oil out of the pan. Check and I'd guess a couple of liters over the full mark so put the one sparkplug and on the 4Y engine the plugs fit in a tube that's sealed to the head with a rubber o-ring, fire it up and think well I'll just drive it into my stall and do some more checking. Well I'm half way to my stall (about 50 feet) and one of the other mechanics is yelling at me your leaking oil, look back and sure enough a trail, get it into my stall and open the cover and check the oil and its' running out the dipstick tube again. Now I know what the problem is so I drain out the oil till there's maybe 2 liters in the engine. Take the PCV breather hose off and get a piece of hose that fits and pull the PCV valve out and loop the hose from vent tube to the grommet for the valve, fire it up and head to the steam pad and when I get there open the hood and there is oil coming out around the spark plug tube where they go in the head. Shut it down and drain the oil till it's at the proper level and steam the oily mess off ( don't worry there is an oil recovery system on the pad drain ) pull the hydraulic oil pump off the engine and driver it back in my stall. Replace the pump and drain and change the oil filter and oil, refill to proper level and replace the plugs and air filter and return to the customer. The unit had about 18,000 hours on it. Still going after an over fill.

 

Good story Frank, I saw something similar on a D4 Cat we had on the farm where I grew up.

I like to keep the oil just a bit below the top bump, but I agree anywhere between the two is OK. If it were more than a cm or so above the top bump I might take some out.

 

My personal rule of thumb is that anything more than 1/16th above the full mark calls for siphoning some off (yes, with a straw).

 

This Forum is awesome! I checked my oil when I got home and it was a hair over the second bump. I was worried about this and so checked the forum. It was great to see this area so well discussed.