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I want to add some information I have culled from recent reading into the synthetic motor oil manufacturers' claims of cleanliness.
Some of these are quoted in the OP in this thread: e.g., Pennzoil Platinum "is dubbed as one of the cleanest oils claiming to be 8% cleaner than Mobil 1, 17% cleaner than Valvoline, and 20% cleaner than Castrol." Pennzoil product literature currently claims Platinum "keeps pistons 40% cleaner than the toughest industry standard" (footnote cite says "Based on ILSAC GF-5 and Sequence IIIG piston deposit test"); for Ultra Platinum the claim is "65% cleaner"; and in its latest magazine (sent to Preferred Customer subscribers), Amsoil now claims Signature Series oil "prevents piston deposits 93 percent better than required to meet the latest API standard" (footnote cite says "Based on independent testing of Amsoil Signature Series 5W-30, in ASTM D7320 as required by API SN specification").
The current API quality grade (SN) is partitioned into two tiers--plain-jane SN and SN Resource Conserving. The requirements for SN Resource Conserving are identical to those for ILSAC GF-5. The Sequence IIIG test is required for certification under ILSAC GF-5 and some earlier versions of the ILSAC standards, including GF-4. ASTM D7320 lays out details of the test procedure, which calls for a reference engine (currently a GM 3.8 L V6) to be charged initially with 5.5 L of the oil being tested and then run for 100 hours under conditions of heavy load and high heat: 3600 RPM, oil temperature of 155° C (310° F), coolant temperature of 115° C (240° F), load of 250 N-m. Then the engine is disassembled and oil consumption, oil viscosity increase, valve train wear, and piston deposits are measured according to a standardized protocol. The results are then compared to the pass criteria for the applicable ILSAC standard.
Weighted piston deposits (WPD) are reported according to what is called a "merit" system in the literature. The possible values, which are dimensionless, range from 0 to 10 and correspond to the surface area of the piston that is clear of deposits. A sparkling clean, brand-new piston has a WPD value of 10. The higher the weighted piston deposit value, the better.
ILSAC GF-4 required a minimum WPD of 3.5. The draft version of GF-5 called for a minimum of 5.0, but in the final version this was watered down to 4.0 because the bulk dino oil refiners (who supply the iffy-lube shops) were having difficulty meeting the higher criterion.
It is my inference that the marketing claims made by Pennzoil, Amsoil, and others are based on percentages of the ILSAC GF-5 WPD pass criterion of 4.0:
Pennzoil Platinum--"40% better"--5.6
Pennzoil Ultra Platinum--"65% better"--6.6
Amsoil Signature Series--"93% better"--7.7
There are a couple of key observations to make here. First, any motor oil with wide commercial availability, no matter how good it is, will leave piston deposits under Sequence IIIG conditions. Second, these conditions are quite extreme, corresponding to about 6000 miles of hill climbing with coolant near boilover, and are hardly ever seen in an engine with good oil temperature control (such as later versions of the 1MZ-FE with VVT-i, but perhaps not the 2007-2011 2AZ-FE with the oil consumption problem).
In my reading I found a couple of additional items. First, someone on BITOG has quoted a WPD value of 5.5 for at least one member of the Mobil 1 family. (I do not know if this comes from an authoritative source or if it refers to current Mobil 1 formulations.) Second, a biosynthetics vendor (apparently trying to break into the motor oil industry) has cited a test WPD value of 8.46 for estolides in API Group III basestocks ("surpasses the piston deposit ratings of full synthetic formulations currently available in the market").