Q: I have two 2006 Mercury Premier Montegos. One is all-wheel drive, and the other is front-wheel drive. The maintenance schedule states 150,000 miles for transmission service.

Should I have the dealer service both cars at 75,000 miles for better reliability? Also, I could at least drain the old fluid out and top off with new.

Another issue: Should I also change spark plugs and oxygen sensors before scheduled maintenance?

— Darlene Stepanic

A: It sounds like your Montegos receive great care and should reward you with reliable service.

I’m a fan of earlier-than-specified fluid and filter changes when the specified interval is 100,000 or more. The drain plugs on each of your transmissions make DIY fluid renewals tempting, but this needs to be approached carefully.

Your all-wheel-drive Montego sports a CFT-30 continuously variable transmission, or CVT, that is very fussy about its transmission fluid, the fill level and how you check it. Replacement fluid needs to be Motorcraft Continuously Variable Chain Type Transmission Fluid (part No. XT-7-QCFT), meeting spec WSS-M2C933-A. I didn’t have any luck finding an after-market equivalent for this $15-$20 per quart blue fluid, and other Ford-specified fluids such as Mercon, Mercon V or Mercon SV are not interchangeable.

When fluid is dropped and renewed on this transmission, approximately five quarts of fluid are added. To correctly verify and set fluid level, the vehicle must be level, engine running and in park. The transmission temperature must be between 109-117 degrees F, which requires a better-than-basic scan tool to check. CVT fluid has different expansion characteristics than normal transmission fluid, making the temperature spec important. A fluid level that’s too high or too low will cause transmission damage. In addition, the dipstick has rather confusing markings that need to be understood and followed.

My On-Demand 5 database shows the CVT transmission requiring a fluid and high-pressure case filter change every 60,000 miles. Sounds like a job for the dealer, or a heads-up independent shop. Steer clear of any facility that can’t recite the specified fluid or procedure, in the driveway, prior to write-up.

Your front-wheel-drive Montego utilizes an Aisin six-speed automatic transmission requiring about five quarts of Motorcraft Premium Automatic Transmission Fluid (part No. XT-8-QAW) meeting spec WSS-M2C924-A. This is a red fluid, perhaps $10-$15 per quart, with a few after-market brands meeting equivalency. Checking/setting fluid level is slightly easier than the CVT, as the transmission needs only to be fully warmed up with the vehicle level, in drive, at specified idle speed. This transmission shows a 150,000-mile fluid replacement interval. I’d do this one early, perhaps at 75,000 miles.

Regarding early renewal of the spark plugs and oxygen sensors, you’d see an illuminated “check engine” light if these parts fail to work properly, and they aren’t likely to lead to huge financial consequences. One could make a case for spark plug replacement earlier than 100,000 miles, before their threads possibly seize. The oxygen sensors are so closely monitored for performance that I’d let the car tell me when replacement is needed.

— Bergholdt teaches automotive technology. Email questions to under-the-hood@earthlink.net.