It may be a surprise to discover what “normal” means to car makers.
When it comes to warranty issues, it would be safest to assume “normal” means a car in mint condition with fresh oil and perfect fuel piped into the engine from the refinery lab next door, and running for weeks non-stop on rollers with the cruise control set at 42 mph so the transmission’s in lockup and the RPMs at 1,100, in a computer-chip-fabrication clean room at a consistent 68°F with humidity of 30 per cent. Anything else could be “severe.”
Go by the car’s dashboard oil-change indicator.
I’m in Canada. All figures are changed from metric: I run synthetic motor oil, Mobil 1. Last spring I changed the oil, and 7,250 miles-and-change later, in early winter, the oil-change computer said change the oil. The computer is set up for conventional oil, so I waited until the indicator hit 0-per-cent remaining.
I sent a sample of the old oil to Blackstone Labs, because I wanted to know the difference between the oil-change computer’s commandment that in my car is set for conventional oil and the length I could extend the oil-change interval with synthetic.
Blackstone replied that the oil had so much of its additives remaining and so few contaminants (listed and quantified) that I should be able to extend the next change to the 9,000-mile mark.
However, this winter has been particularly brutal, with –20°F to –35°F for weeks, then months, on end. The oil-change computer is counting down the percentages to the next oil change much, much faster than it did during the summer — it really is severe service — so I’m not going to wait any extra time to change the oil, despite it being synthetic. Any extra mileage I could achieve would be shortened, as well. But by how much? Who knows? Blackstone would, but the lab’s results aren’t instant and cost money.
The oil-change doohickey computes the oil-change interval not by any direct measurement of the oil but by the engine’s temperature over the long term, whether it achieved proper warm-ups, the ambient temperatures, the length of trips and so on (the driver’s manual is in the car, so I’m not going out to read it).
Blackstone’s confirming my guesstimate and the computer’s faster oil-change-countdown rate during this very cold winter give me more confidence that the computer’s oil-change countdown is accurate.
It may be worth mentioning that I don’t use the cheapest oil filters on the shelf.
The forums at bobistheoilguy.com (I am not a member and have nothing to do with it) provides lots of information.