I agree with the first part of your statement, and I am aware of the importance of keeping enough oil in an engine. I didn't mean to make it sound like it was the first time I've checked the oil since the oil change, I just don't do it as often as I would like because of the tape hassel. Fortunately the car doesn't consume much oil so I can check it less often :) .
The second part is somewhat debatable (FE suffering from low oil level), a full crankcase will have more windage than one with a lower level of oil. Windage = energy loss = FE slightly lower. I would rather have a crankcase in the full range though to be safe especially since mine only holds 3.7 quarts or so.:) To avoid problems I would never drive with the oil level lower than the low mark on the range. But maybe you meant if the oil were so low it weren't pumping and cooling properly, then yes FE would suffer from the higher friction and temps etc as the engine begins to fail :o .

my manual asks that you change oil ever 7,500 miles, or 6 months, the synthetic I use says 25,000 miles, or once a year, so I do it once a year, of course I also have the smallest micron oil filter I could find, and it's twice as long (more filter area) then the stock filter, and I did an oil flush when I bought the car.

I just posted this on another thread, but I think it applies here also.

I like that Mobil 1 is finally pushing for extended oil drains using the extended performance oil line. The only thing that I don't like is that if your car is still under warranty than you have to follow the oil life sensor or owners manual. But if it's not under warranty, your covered for 15,000 miles or 1 year. Does this even make sense? Also it says if your operating under sever conditions to change your oil as stated in the owners manual. I think it's so they can weasil out of paying up, by saying your car was used under severe conditions so your SOL. Here's the warranty info for Mobil 1 Extended Performance.

This is a limited warranty covering the Mobil 1 Extended Performance lubricant you purchased. ExxonMobil warrants its lubricants to be free from defects and will replace any Mobil lubricant that is defective. For vehicles covered by a warranty, follow the vehicle's oil life sensor or the oil change interval recommended in your owner's manual. Follow your owner's manual if the vehicle is operated in any of the following severe services: racing or commercial applications including taxis, limousines, etc.; frequent towing or hauling; extremely dusty or dirty conditions; or under excessive idling conditions. If your vehicle has exceeded the duration of the manufacturer’s warranty, ExxonMobil warrants that the Mobil 1 Extended Performance lubricant you purchased will protect your vehicle’s critical engine parts from oil related failure for 15,000 miles or 1 year, which ever comes first. If not, provided the engine was serviceable at the time the oil was installed, Exxon Mobil will have your vehicle repaired and replace the oil at no cost to you.

I also do a flush on the first oil change. And also do the tranny at that time also.

I don't know why it happens (although I'm guessing it's because the oil doesn't lube as well when the level is low). However, I have noticed the effect in my wife's Civic. She seems to have an uncanny ability to let the oil level drop too low (I am trying to teach her to read the dipstick), and I've found found the dip-stick at various stages of low (including about 2.5 quarts down, when she had an oil leak at the drain plug, that she wasn't aware of). From what I can tell, the FE in the car started dropping as soon as the oil level was about 1/2 quart down, and kept dropping the lower the oil level got. Which is why I try to help her keep her oil level up much closer to the "full" line on the dipstick.

I think maybe you are right, because when the oil is low it begins to suck in air = cavitation = higher friction = less surface area served by the air infused oil

Ever wonder why Mobil is pushing an 'extended drain' of 15,000 miles, on their oil that was originally rated for 30,000 miles service life?

The folks who started the whole thing forced them to. It's the same as your laundry soap now being 'consentrated' and bio-degradable.
In both cases two companies with nothing in common but similar sounding names invented superior products, almost thirty years befor the main stream companies were forced to respond. Amway with soap and Amsoil with the first automotive synthetic motor oil.

For 30 years it has been possible to go well beyond the conventional oil change times.

Some of us used Amsoil oil and filtration to go over 100,000 miles without doing more than changing the spin-on filter and adding some oil.

Today you can simply use Amsoil oil and the Amsoil EA oil filter and change both at their rated time of 25,000 miles or one year.

So, ya gotta ask yourself, punk; do I want the longest engine life and best lubricant performence, or not? Well, do ya?

I use full synthetic 0w40 in the cavalier and change it every 10k. I get a sample tested every change and it still has active additives left.

In the Scion we change it every 3k and use whatever the stealership dumps in it.

i change mine in my truck every 3-3.5K as i do haul a trailer quite often and have loaded the bed up with crap alot.

the car usually 3-4K. usually i do them at the same time so i don't have to drag all the oil change crap outa the garage twice.

it could be like the Model A had 500 mile intervals :P most people now go at least a 1000 miles before they change.(remember there is no filtration oil system at all in these engines, but also have very large oil "ports" if u will call em that, its a splash system/gravity feed kinda. as the oil pumps job is to oily pump oil up to the valve chamber then it drains back down.

I do 5,000 miles/1 year (whichever comes first) on all my vehicles, that way I know if my mileage is divisible by 5,000 then it needs to be done. Here are the flavors I'm currently using...

98 GMC K1500, 5.7L V-8: Mobil1 0W-30
86 Chevy C10, 5.0L V-8: Mobil1 10W-30
81 Buick Regal, 3.8L V-6: Mobil1 5W-30