Back in 2006?, Honda came out with the HTO-06 (Hot Tube Test) which tested oils for turbo charger performance for deposit prevention. At the time few oils could meet it. You often see ASTM D6335, also known as the TEOST 33C test, used by some brands to compare deposit protection. This article is from 2016. Toyota, like Honda, found the test to be severely flawed due to the fact that it does not test used oil. Used oil has fuel contaminates in it. It is insolubles that are the main culprit of lower coking start temperature which cause deposit formation. Clean/new oil can be considered a value testing measure. Link Highlights of article:
The Toyota team also conducted laboratory tests to look at when coking happens and under what conditions, evaluating both used oil and fresh oil. They found that key was the presence of insoluble materials, which in gasoline engine oils, were found to be polymerized hydrocarbons with carbonyl and ester functional groups. Lubricants with even small amounts of insolubles formed deposits relatively quickly at anything above 180o C. Those free of these external materials did not coke, even at significantly higher temperatures. For instance, fresh oil free of insolubles started to coke at 280o C, a full 100o C higher than used oils with insolubles in them.
Since there was a big difference between the coking start temperatures with the used oil and the oil that was oxidised in-lab (coking temperature is well above 200° C), Toyota was able to determine that insolubles are the main driver of lower coking start temperature. Because the starting point is degraded fuel that’s been through combustion, fresh oil doesn’t exhibit anything like the temperature sensitivity and problem that used oil does.
Deposit formation is not as simple as one may think. Over the years I've noticed via Amsoil's competitive oil testing, that Mobil 1 and a few other brands perform worse today on the TEOST test than they did back in the early 2000's. Currently, if you go to Mobil 1's website, they claim 2x better turbo performance than a competitive synthetic oil (regular Mobil 1) in turbo deposit protection. This was done using the Hot Tube Test as noted on their website. Now I understand why there is this discrepancy and also know what has more value in determining real world deposit protection. Mobil 1 Hot Tube Test Mobil 1 was head of the curve on turbo deposit prevention and LSPI. You're going back to 2006 and 2010. One of the most important things an oil has to do in a turbo charged engine is not clean deposits, but prevent them in the first place.
Toyota researchers also looked at screening tests to detect turbocharger coking, such as ASTM D6335, also known as the TEOST 33C test, which has been used in some of the ILSAC engine oil specifications. As already mentioned, the Toyota researchers found that the presence of insolubles derived from degraded fuel molecules was the key factor to reproduce the turbocharger coking phenomena. The TEOST 33C test does not involve it. Therefore, they do not think it is a good engine oil-screening test for turbocharger coking.