Howdy. It's true, used oil analysis is to monitor the oil condition so as to get the most out of it. Some diesel fleets are easily up to 40,000 to 60,000 miles and more, based on watching what happens. Doing this is safe as long as the oil type, service conditions, and engine all stay the same - which is fine for on-road highway fleets. Not applicable to pass car use.
GTS Bruce is therefore correct - change your oil often enough and the measurements indicate more of the bounce in the analysis results rather than what's happening in the engine.
Viscosity (that second number, after the "W") should be between 9.3 and 12.5 for SAE xxW-30, between 12.5 and 16.3 for xxW-40. Yes, there's quite a range, it's not a single target value.
Fuel dilution and glycol, measured at the $20/sample labs, are often not accurate or sensitive below 0.5%. You may get results of 0%, 0%, and 0.6%, which only means the first two could have been 0.4%, but the instrument used couldn't detect it.
Glycol level is something to look for if you suspect the dreaded head gasket leak.
Geoff's used oil wear metals levels are low, as you'd expect with 1000 mile samples. Iron levels even up to 100ppm at the end of drain are probably OK. Others in the fleet business (remember this is gas, not diesel) may have different values.
Zinc-phosphorus additives are the anti-wear components. The legal limit for ILSAC GF-3 oils is 1000 ppm phosphorus because it may affect catalyst life. The next engine oil category has proposed to cut this to much, much lower. In my opinion, this is a risky move to lower the antiwear additive that engines were designed around, but OEM pressure will eventually force this through. This is why I advocate diesel oils, since they don't (yet) have catalysts and may only have particulate traps - so there's no restriction on the amount of phosphorus in a diesel oil. Geoff's oil shows about 600 ppm. This is why I say that passenger car oils are only "OK" and the kind of oil I would change often. The thing everyone should think about is that this "OK" additive package is put into "superior" synthetic oils. The combination is not necessarily "superior."
By comparison, Steve's oil shows 1300 ppm phosphorus. This disqualifies the oil from claiming ILSAC GF-3 because it is over the phos limit. It can still claim API SL, since it would still meet the engine performance requirements. If fact, other aspects of Steve's oil would hint at a diesel type additive package in a passenger viscosity grade. The high calcium level indicates a high detergent amount and therefore high ash, all diesel characteristics. Add the magnesium, another indication of a detergent additive, and Steve has robust protection against piston ring, ringland, and varnish deposits. This is proven by the pretty good TBN (a sign of how much detergent is left active) after 6000 miles. Most passenger car oils only start at 7 TBN and go down from there.
Despite Steve's horseplay, the average oil temperature must not get above 200F that often, since there's 1% fuel showing. It'd be more typical in the summer to see less than 0.5% Unfortunately, there are so many ways to indicate the level of oxidation, I won't even guess as to how to interpret "2.2% oxidation." Since the result of oxidation is thickening, you might as well look at the viscosity.
One thing you all might want to do is send a new oil sample periodically. That quart bottle may stay the same on the outside, but the fluid inside will change every two or three years.
Soapbox: Diesel oils have lots of good additives. It's the additives that protect against wear, keep the engine insides clean, and control the viscosity of the oil (keep it from going up due to thickening or down due to shear.) Yes, they mainly come in 15W40 grade, but there are 5W40 synthetic diesel oils (Mobil Delvac 1, Shell Rotella T Synthetic, and others) that are about $18/gallon at Walmart. OK, I'm done.
