10-30W versus 10-40W

[Farmerford]

Bob:

The issue may not be viscosity, but rather resistance to thermal breakdown. Petroleum oils (unlike synthetics) become mulit-weight by the addition of long string polymers that curl and uncurl according to temperature. A 10W-20 oil requires few long string polymers, while a 10W-40 requires a much larger quantity. The polymers are subject to thermal breakdown at high temperatures. When they break down they form a thick goo that blocks oil passages, etc.

In the early 1980's many of the then new small displacement engines put into larger cars for fuel economy operated at much higher internal temperatures than did the earlier slow turning large engines. It was common for 10W-40 oil to get hot enough in these cars to undergo thermal breakdown and turn to syrup. For example, I had a 1993 Isuzu Rodeo that prohibited 10W-40 oil and voided the warranty if it was used.

Honda may be concerned that elevated temperatures in the air cooled engine in the pressure washer, which works pretty hard if the washer is used continuously, could be high enough to cause thermal breakdown in an oil such as 10W-40 containing the large quantity of long string polymers needed to create the wide range. In contrast, Honda may have determined that 10W-30, with fewer (and perhaps different) long string polymers will tolerate the higher temperatures.

If you read some of ads for wide viscosity range oils you will see frequent references to "resists thermal breakdown", suggesting to me that the problem is still around. Today's wide viscosity range oils may be less prone to thermal breakdown than those of the 80's and 90's, but Honda's requirements suggest that the problem has not been completely solved.

[jayhaitch]

I remember reading somewhere (but can't remember exactly where....) that a petroleum 10W40 turns into a 10W30 in a relatively short time by breakdown of these viscosity index improvers. I seem to remember something like 3 or 4,000 miles in a car. Not sure how this would translate into hours on a pressure washer.

That's why old outdoor power products specified straight 30 weight. No VI improvers to break down, less sludge, less oil passage blockage, assuming the engine is used in warm weather only.