The PC-11 product that covers the 15W-40 and traditional viscosities should be backwards compatible, but the product intended for fuel economy may not. There is talk of tightening the limits, where you may have both light and heavy 30 weights. “It is those light 30 weights that might not be backwards compatible,” says Arcy.
Due to concern over the compatibility of previous engine designs with the new light oil classifications, backwards compatibility may become a question for each engine manufacturer to determine. “The engine manufacturer may say it is backwards compatible to 2007, but not anything before 2007,” says Arcy. “Those are the kinds of details that have to come out yet.”
McGeehan adds, “It may turn out that, even as we lower the oil viscosity with the severity of these engine tests that are under development and improvements in the oil, it is a possibility it would be backwards compatible, as well. But we are not that far along.”
Regulations have reached a point where compromises to make an oil backwards compatible just might not be acceptable. Higher performance oils that can yield a 1.5% fuel economy benefit justify the decision to market more than one type of oil for heavy-duty diesel engines. Things may get a little more complicated for large fleets, but there is a measurable payback.
“We have been lucky that the engine oils we design have always been useful in the previous engines,” says Arcy. “That may not be the case anymore. We may have to actually draw a line in the sand and say anything from this year on uses these engine oils and anything from this model year takes these. If we can achieve an extra 1.5% or 2% in fuel economy, long term it is going to pay off.”