Q: I own a 1998 Toyota 4Runner. The owner’s manual calls for the use of Dexron III transmission fluid. I normally do a quick drain and refill of the transmission fluid every 12,000 miles. All the transmission fluid I now see on the market is labeled for both Dexron and Mercon use. Is it safe to use this fluid in an automobile that uses only Dexron III fluid?

A: Good question! This is a confusing and controversial subject. Dexron III was General Motor’s recommended automatic transmission fluid from 1993 until it replaced it in 2006 with Dexron VI and suspended licensing agreements for oil companies to make and sell III. Quite a few other vehicle manufacturers adopted the use of Dexron III fluid as well. Mercon was Ford’s fluid of choice from 1987 through 2007, when it was replaced by Mercon V and licensing for the older Mercon was suspended.

Dexron III and Mercon are similar enough that many oil companies market the multivehicle fluids you’ve seen, which no longer have manufacturer oversight. Some in the industry are fine with this, while others express concern. Dexron VI has many significant improvements and is backward-compatible with Dexron III transmissions, although GM stops short of recommending it for other manufacturer’s vehicles. Yikes!

I consulted with my transmission guru, John Strenfeld, about what fluid is best for your 4Runner. The first thing he said was, “How large is his wallet?” I gulped, he chuckled, and said, “Dex VI is the coolest thing ever except for the high price (about double that of Dexron III/Mercon)!”

John’s favorite feature of Dexron VI is its greatly enhanced viscosity stability, which is important for electronic control solenoids to function reliably. You can add the new fluid incrementally or all at once. Don’t forget to renew the in-pan transmission filter periodically.

Q: I have a 2003 Chevrolet Silverado showing a particular diagnostic trouble code. I have changed the gas cap, the gas tank sensor, both vent valve solenoids, the charcoal canister and passed a smoke test. Would a Chevy dealer have a more detailed tester to isolate the fault?

A: The dealer’s service info may be more important than its tools! GM issued a service bulletin in 2008 addressing trouble codes occurring in 1999-2008 trucks, particularly those driven in dusty conditions.

The bulletin recommends, after confirming a faulty vent solenoid issue, installing a service kit. The kit is a replacement vent solenoid that is sealed, mounted in a new location and draws air from a remotely mounted filter box. Since you’ve already renewed the vent solenoid with no success, it’s possible dirt has built up in the vent circuit hoses. They should be flushed out or renewed.

The aforementioned components are part of the evaporative emissions system found on 1996 and newer vehicles. Fuel tank vapors are collected, stored in a charcoal filled canister and purged (consumed by the engine). In order for successful purging of the stored vapors, a vent allows fresh air to enter the canister, allowing a flushing action. The vent valve closes when the system routinely checks for leakage and purge flow. Your truck appears to have a restriction in the vent plumbing.