JC, I don't see the advantage of running the sump lower on oil as a method of cooling oil. If the sump has air blowing against it, then the only oil which will be cooled will be the oil in contact with the sump, and that would indicate keeping the sump full. The air above the oil level in the sump will not be assisting in oil cooling...in fact it's mostly composed of hot blow-by gases and the less amount of room for that (in other words a full sump) the better.
I'd bet that most of us, when checking the dipsticks of our cars just before a long highway trip, will add a quart of oil if it reads a quart low. In fact, I'll bet most of us would even consider splitting a new quart bottle, just to top it off to the full mark for such a trip, because we all want our cars to last a long time and not miss a single drop of that precious lubrication on a long, hot, summer trip. Yet, our cars have liquid cooling and run at more even temperatures and virtually never suffer risk of overheating the oil. And even if we did run low and damage an engine, all we have to do is pull over to the side without a lot of risk to our safety. But we're discussing leaving on an airborne trip in our precious airplanes with less than what we give our cars?
At the risk of starting something else, (putting on my Nomex now) I also believe it to be an old wives tale and a poor practice about departing with the oil level low to avoid blowing oil out the breather.
First of all, the breather outlet is at the TOP of the crankcase, well above the sump and well above the crankshaft and all other moving parts in the engine. The oil that leaves the breather is, at the most, misted by all the thrashing that goes on in there from "sling" off the crankshaft. That oil then can be blown out the beather
if excessive gas-pressures, improper breather elbow placement, or both exist...but neither of those conditions have anything at all to do with the amount of oil in the crankcase sump....because even a full sump (8 qts) has the oil level well below the crankshaft throws...and an 8 qt/full oil level drops even further when the engine is running due to some of that oil being "in transit" throughout the engine's lubrication systems, i.e., the rocker boxes, pushrods, gear/accy case, etc. (And it matters not that if/when the oil level may be intentionally kept low...oil still slings off the crankshaft and blow by gases still flow out that breather tube, so if that is the problem then keeping the oil low will not arrest it.) I operate my engine with 8 quarts, and when it gets below 6.5-7 qts on the dipstick I add a quart. My oil consumption hovers around one in 10 hours, and I don't see any evidence of "excessive" beather blow-by. (A couple of two-bit sized spots will fall out of it onto the ramp about 15 minutes after shut-down sometimes, but I consider that normal.)
The seaplane SHOULD have a different dipstick if the operator wants it to read accurately. But it's not required, nor is it even common.* (Most seaplanes (170's) were/are not equipped with the 172/tricycle-gear dipstick because the floats were considered an alternate gear on a standard 170. Lots of operators switched the airplane back and forth, and besides, many 170's actually were float-equipped by an aftermarket STC rather than by Cessna. Consequently many float-equipped 170's operated with the standard conventional-gear dipstick installed however, and overfilling a sump will lead to frothing and overheating of the oil, hence the Owner's Manual comment.)
*In later years Cessna's very successful model 172 far exceeded the production output of the 170. Consequently many more O-300 engines were produced for a tricycle stance than a conventional stance. The engines have been swapped around on airframes almost without regard to the type landing gear utilized. Therefore it is very common to find 170's on conventional gear with the INCORRECT tricycle gear dipstick in them. This results in the engines being run with LOWER than intended oil levels. This situation also results in those seaplanes using conventional-geared dipsticks being subject to OVER servicing with oil, which leads to frothing and overheating...therefore it's necessary to run a seaplane which has a conventional-gear dipstick with an oil level that READS about 6....even though it has 8 quarts of oil in the sump making that reading on that dipstick.
TRIVIA: Keep in mind that the mfr considers these engines airworthy with oil consumption rates almost a whole quart per hour! (at max continuous) And their engineering/maintenance data state that. They also state the MINIMUM oil level for safe operation to be 4 quarts. So, in order to avoid potential engine damage, only if a sump is filled with 8 qts of oil can it fly for 4 hours (the average endurance of the airplane at that power setting) at the maximum allowable consumption and still land with sufficient oil to avoid damage to the engine.
(Some aircraft designs have such endurance that oil sumps must be enlarged for certain engines to accomodate certification criteria. This usually results in a "dash" number/designation change for the basic engine model.)
