Astrochimp,
I've got the report in front of me, and I'll quote selected passages. You're right, they didn't do many cold starts. This is from the July 1996 edition.
"We put identical engines with precisely measured parts into the cabs at the beginning of the test, and we changed their oil every 6000 miles. That's about twice as long as the automakers recommend for the severe serice that taxicabs see, but we chose that interval to accelerate the test results and provide worst-case condition. After 60,000 miles, we disassembled each engine and checked for wear and harmful deposits."
"When the cabs aren't on the go, they're typically standing at curbside with the engine idling - far tougher on motor oil than highway driving."
"Big-city cabs don't see many cold start-ups or long periods of high-speed driving in extreme heat. But our test results relate to the most common type of severe - stop-and-go city driving."
"We tested oils of the two most commonly recommended viscosity grades - 10W-30 and 5W-30."
"A popular belief is that 5W-30 oils, despite their designation, are too thin to protect vital engine parts when they get hot. However, one of our laboratory tests measured the viscosity of oils under high-temperature, high-stress conditions and found essentially no difference between 5W-30 oils and their 10W-30 brand mates. But at low temperatures, the 5W-30 oil flowed more easily."
"More extensive tests, under other driving conditions, might have revealed minor differences. But thorough statistical analysis of our data showed no brand - not even the expensive synthetics - to be meaningfully better or worse in our tests."
"All the oils proved excellent at preventing sludge. At least part of the reason may be that sludge is more apt to form during cold startups and short trips, and the cabs were rarely out of service long enough for their engine to get cold. Even so, the accumulation in our engines were so light that we wouldn't expect sludge to be a problem with any of these oils under most conditions."
"In our tests, brand didn't matter much as long as the oil carried the industry's starburst symbol."
"One distinction: According to the laboratory tests, Mobil 1 and Pennzoil Performax synthetics flow exceptionally easily at low temperatures - a condition our taxi tests didn't simulate effectively. They also had the higher viscosity under high-temperature, high-stress conditions, when a thick oil protects the engine. Thus, these oils may be a good choice for hard driving in extreme conditions."
"None of the tested oils proved better than the others in our tests. There may be small differences that our tests didn't reveal, but unless you typically drive under more severe conditions than a New York cab does, you won't go wrong if you shop strictly by price or availability. Buy the viscosity grade recommended in your owner's manual, and look for the starburst emblem. Even the expensive synthetics worked no better than conventional motor oils in our taxi tests, but they're worth considering for extreme driving conditions - high ambient temperatures and high engine loads or very cold temperatures.
On the basis of our test results, we think that the commonly recommended 3000-mile oil-change interval is conservative. For "normal" service, 7500-mile intervals (or the recommendation in your owner's manual) should be fine. Change the oil at least that often to protect your engine and maintain your warranty. Even for the severe service experienced by the taxis in our tests, a 6000-mile interval was adequate. But some severe service - frequent cold starts and short trips, dusty conditions, trailer towing - may require a shorter interval. Note, too, that special engines such as diesels and turbos, which we didn't test, may need more frequent oil changes."
So there you have it, straight from the horses mouth. I had to key all this in, so there may be some typos.
Oils tested were:
Castrol GTX 5W-30 and 10W-30
Exxon Superflo 5W-30 and 10W-30
Shell Fire & Ice All-Season 5W-30 and 10W-30
Havoline Formula 3 5W-30 and 10W-30
Mobil 5W-30 and 10W-30
Pennzoil 5W-30 and 10W-30
Quaker State 5W-30 and 10W-30
Valvoline All-Climate 5W-30 and 10W-30
Kendall Superb 100 10W-30
Mobil 1 10W-30
Pennzoil Performax 10W-30
Valvoline Semi-Synthetic DuraBlend 10W-30
Whew...
. The explorer gets whatever the hell I find on sale, LOL j/k.
