Is Saab joing the ranks of other sludgemobile makes like Mercedes, Toyota, VW, Audi, and Chrysler, or is it a more general problem of marketing overriding engineering when makers specify long oil chnage intervals ? Oil sludge is something that use to be common, in fact expected in the older VWs, a vehicle that I use to work on part time for extra cash. It had a small sump, high operating temperatures, and no oil filter to speak of. I added a larger oil cooler and a full flow oil filter, which also increased the oil capacity significantly, did 3k oil changes, and the ex (who ended up with the car) said that every mechanic was amazed at how clean it was. The last time I talked to her it had almost 140k miles and was running like new.
Now days people seem to expect long oil change intervals, whether it's warranted or not, and we've seen literally millions of vehicles end up with oil sludge problems, some severe enough to produce engine failure at 60k or less miles. Spec'ing dino 5W-30 for 7500 mile oil changes (people typically don't read the fine print about severe duty use) seems like a recipe for potential problems, and the results indicate that it is. Synthetic isn't going to solve all of the problems, as with some usage the oil needs to be changed because of soot, high silicon from dust and such, water, etc., things that filters don't often remove. One of our cars is on a low use schedule as we try to avoid having two or more vehicles wear out at the same time and require replacement, so it probably sees 5k to 6k miles a year. With a '15k' mile oil should I replace it once every three years ? I think that regardless of what I put in it a six month change is reasonable, considering the potential for condensation here in the Pacific NW and the low usage. With my diesel truck soot is driver for the OEM oil change interval, something that a synthetic won't solve.
In summary, as I noted previously, if you don't KNOW the condition of your oil and you hope to keep the vehicle for awhile, long oil change intervals are a gamble, one with little benefit considering the cost of oil.
http://www.caranddriver.com/article.asp?section_id=27&article_id=8794&page_number=1
Other diligent owners changed their oil, but as it turned out, not often enough. Like most owner manuals, Toyota's specified oil changes at 7500 miles. But its manuals also specified 3750-mile changes for cars in "severe" service. "Severe" service applies to vehicles used on short trips, especially in winter, situations where the engine never warms up. It also involves towing and stop-and-go driving in heavy traffic, especially in hot climates, which elevates oil temperature. These are exactly the conditions that promote sludge formation.
So how is an owner to determine the threshold between "normal" and "severe" service? Toyota concluded that an owner couldn't be expected to know and has since revised its oil-change schedules for all its vehicles to 5000 miles.
