***"Contaminants and moisture should already be entrained in the oil, shouldn't it?"*** To a degree, yes. Contaminants will be entrained during operation of the engine and circulation of the lube oil. Moisture is a "transient" contaminant with a short life span. Even though the H2O is "transient", some of the moisture will re-combine with other components of combustion even while still in the combustion chamber (prior to being "blown-by". These compounds are NOT TRANSIENT and will remain in solution in the oil where they will become acid and attack the oil and engine surfaces. Actual moisture is introduced to the crankcase lube oil in 2 ways. Most of the moisture gets into the crankcase as a component in the blow-by gasses during cold start up. The water is one by product of the combustion process. The other, and "lesser" way is from the natural humidity of the inducted air. The "inducted air" moisture gets to the crankcase lube oil as a component of blow-by, but also in the air that is "fed" to the engine to ventilate the crankcase (PCV system). The moisture is only able to mix with the lube oil when the engine surfaces and the oil mass are cold. After the engine and oil mass heat up, most of the moisture is "driven out" of the oil. The evaporated moisture will be drawn out of the crankcase by the PCV system and be sent back through the induction system again where it can be expelled out the exhaust. High temps are the determining factor in keeping the moisture "out of solution" or "out of emulsion" in the lube oil. Even though the contaminants are in suspension during operation of the engine, as soon as the engine stops and the majority of the oil drains to the sump, STRATIFICATION of the liquid begins to happen. During stratification, the heavy, solid particles settle to the bottom of the sump first. After that, the liquid components of the oil begin to separate into distinct layers, or "strata", according to the specific gravity of each component of the blend. The longer the time at rest, the more complete will be the settling process. The "more used" or older the oil is, the faster the solids will settle too. ***"Sludge is another thing, and again I doubt you'll get much more of that by running the motor. The oil pump isn't going to pull it off the bottom of the pan (at least you'll hope not) and you'll pretty much have to remove the pan to clean it out....or run some solvent thru your motor first."*** The sludge is formed ONLY WHEN the oil in the crankcase has remained in service for too long. Oil that has been in service too long, has lost most of its additive package to heat and contamination from blow-by. When good oil is fresh or new, hardly any settling or stratification occurs (unless the engine is 'off" for a very long time). During the time after fresh new oil is put in an engine, practically everything the oil has "picked up", remains in suspension. If the engine has always had "timely" oil changes, and has always been run long enough after cold starts to drive off moisture, there will be very little (if any) "sludge" in the pan or elsewhere in the engine. As the oil ages and degrades, stratification begins to occur during periods the engine is not running. During this "phase" of the oil's life cycle, any particles that settle (sludge keystones) will be "scrubbed" and swirled back into suspension by the detergent additives in the oil's formula. As these additives grow weaker and thinner with continued use of oil that is degraded (and really no longer fit for service), the "sludge" that forms during "off periods", is more difficult to "remix" in subsequent run periods. So, consider the oil additives as though they are "continuous solvents" that scrub and clean the engine whenever the engine is running. Oil change is good. Fresh oil is good. Vibrant additive package, good. In my opinion.....there is a definite benefit to changing the oil WHILE STILL HOT from a long run time. The "perfect time" to change the oil would be at the end of a 12 hour period of use, immediately after the engine was turned off. Obviously, this is not practcal, so we must try to do the next best thing.

917.289031 is the model number. Like I said in the original post, the engine is not the original but a replacement...just so you know. I just took the spark plug out and cranked it and that allowed the starter to spin the flywheel constantly. Exciting! What do you suggest?

Here is a little more information that might help clarify the purpose of the diode across the PTO coil (paraphrased from a Wikipedia article): The functional name given to the diode is a 'snubber'. Snubbers are frequently used in electrical systems with an inductive load (PTO coil) where the sudden interruption of current flow often leads to a sharp rise in voltage across the device creating the interruption (the PTO switch). This sharp rise in voltage is a transient and can damage and lead to failure of the controlling device (PTO switch). When the current flowing is DC, a simple rectifier diode is often employed as a snubber. The snubber diode is wired in parallel with an inductive load (such as the PTO coil). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load (PTO coil) is rapidly interrupted (by the PTO switch), a large voltage spike would be produced in the reverse direction (as the inductor (PTO coil) attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load (PTO coil) allows the current from the inductor to flow through the diode rather than through the switching element (PTO switch), dissipating the energy stored in the inductive load over the series resistance of the inductor (PTO coil) and the (usually much smaller) resistance of the diode. So, this inductive kick does not feed back into ground or into any other part of the engine such as the starter (as mownie correctly mentioned). The potentially damaging energy is dissipated in the series resistance of the diode and the PTO coil.

Command has hydraulic lifters instead of the solid tappets of the Courage. Hydraulic lifters permit the valve train to operate safely with "zero lash" between the valve stem and rocker arm. This equates to a quieter engine, and generally more efficient flow through the induction and exhaust systems. Twin engines have smaller combustion chamber components than a single with the same displacement. The smaller, lighter parts just naturally cause less noise, and the power overlap of of a Vee engine attenuates a lot of the heavy thump of a single.