Blenders often express frustration that they cant routinely inter change base stocks from different manufacturers slates. After all, isnt a Group II a Group II, whoever makes it? Not quite – and not as currently accepted by lubricant industry bodies such as API in the U.S. and Atiel in Europe, when comparing the same base oil group and even viscosity grade but from different manufacturers slates.
As managing bodies, API and Atiel define a slate as a product line of base stocks that have different viscosities but are in the same base stock grouping and from the same manufacturer. Feedstocks, processes and manufacturing locations for a given slate can be varied, but not manufacturing specifications. Within a manufacturers slate, base stocks are fully fungible.
Before slates existed (about the mid-1990s), there was no constraint on interchange of base stocks, until it was realized that crankcase lubes exhibited field performance issues.
This was mainly the result of crankcase additive technology having marginal performance and being highly de- pendent on base stock quality. Thus, finished lubricants manifested performance issues when the development base stock was changed to a weaker one.
Today, both API and Atiel have protocols that not only control base oil interchange (BOI) within and between slates, but they also actively manage the process so that where possible, only the minimum number of engine tests from a performance category need be repeated when base stock slates are changed.
The basic concept of BOI protocols is that when changing base oil slates, an engine test should be repeated un- less the managing body has demonstrated, on a statistically significant basis, that there is no base oil appetite. Base oil appetite is more a term used by crankcase lubricant formulators than a formal definition, but it means that there is a significant contribution of base stock to the performance of the lubricant, for which the additive technology cannot compensate. When this is the case, it makes obvious sense that the engine test with that base oil appetite is repeated to demonstrate no harm when base stocks from different manufacturers slates are inter- changed. When there is no appetite, that specific engine test can be waived for BOI purposes only.
Within API as a managing body, when new crankcase performance categories are developed, part of the development process involves running matrices of engine tests. This is done using experimental design, both as
a function of base oil slate variation and with limited additive technology chemistry variation, such that statistically significant data is generated to determine whether there is a base oil appetite for a given engine test.
Of course, running engine test matrices is very costly when even a single engine test can cost many tens of thousands of dollars. Typically, tests can only feasibly be run on the basis of cross-industry collaboration and funding. For API performance categories, this involves contributions in cash or kind from most interested stake- holders: OEMs, additive companies, lubricant marketers, etc.
However, once the matrices have been run and analyzed, and base stock appetites confirmed or discounted for a given engine test, much repetitive testing is avoided as base stock slates are varied for given additive technologies. BOI typically requires specific engine test repetition either because the engine test matrix demonstrated a base stock appetite or because no base oil interchange matrix has yet been run.
Whats the future of BOI? Recent years have seen some evolution of the basic BOI process through so-called single technology matrices that have been developed under API guide- lines. Here, specific additive technology products – i.e. a given additive chemistry, treat rate, and usually specified SAE viscosity grades and viscosity modifier chemistry – have been demonstrated to have no base stock appetite for a chemically book-ended range of base stock chemistry. This is mainly the domain of additive companies who want their specific technology
to have as large a range of base stock slate coverage as possible. This option is, of course, highly additive package-specific and while it is a welcome addition to
base stock fungibility it does not help when the additive chemistry, treat rate, viscosity modifier or additive sup- plier changes – even for the same performance claims. Further evolutions of the single technology concept can be studied in the most recent API 1509 release, which gives some additional base stock fungibility.
Another aspect that may deserve some consideration is whether some of the base stocks coming on the market today are becoming progressively more similar. For example, how different can base stocks really be when some of the higher VI base stocks use largely similar hydrocracker-derived feed stocks that are highly processed with similar catalysts and with significant ring cracking and isomerization? These processes effectively disconnect the base stock from the original feedstock composition. Where differences occur between engine test results for base stocks from different manufacturers – even within the same API or Atiel Group – is this just a manifestation
of the limits of engine test precision or a real base oil appetite? Maybe, this is something to consider in more detail for the future.