Q I have a '97 Nissan Sentra with 160,000 miles on it. I last changed the transmission fluid at 70,000 miles, but at 110,000 miles I was told that changing the transmission fluid with a car with that many miles could cause problems, so I didn't change it.
I've never had a problem with the transmission until last week when it slipped into neutral for about 10 seconds while I was accelerating out of a line of traffic. The service provider I talked to highly recommended changing the fluid -- with the caveat that it could exacerbate a problem if one exists. He said that the fluid deteriorates over time and that it should be changed.

Because the slipping happened only once and during high temperatures, I am wondering whether I should follow his advice or leave well enough alone. Could high temperature have anything to do with it? Yesterday with temps in the mid-90s it seemed like it might have slipped a little, but not like the first time. Thanks for your advice.

Your service provider gave you exactly the right advice. In my opinion, you should flush and refill the transmission with 100 percent new fluid and a new filter for the simple reason that this is the only service you can do that might eliminate the slippage.

But let's back up a bit. You were given bad advice at the 110,000-mile mark. Why? Because the transmission fluid at that point had only 40,000 miles on it! It should have been changed at that point or shortly thereafter as part of routine maintenance. Now, at 160,000 miles, the fluid is oxidized and contaminated with 90,000 miles of driving on it, and coupled with a total of 160,000 miles of wear and tear, it might be contributing to the slippage you've experienced recently.

Automatic transmission fluid is fundamentally a very high-quality 10-weight oil, with a very high level of detergency to help keep it clean for tens of thousands of miles. Even though transmission fluid is not subjected to combustion temperatures and byproducts, it is heated to 200-plus degrees several times a day, day after day, month after month, year after year. The fluid is subject to oxidation, which can create varnishing that can restrict fluid flow through the complex hydraulics of the transmission. A complete flush with 100 percent new fluid and a new filter will hopefully remove any existing oxidation and varnish, and restore performance of the fluid and transmission.

The idea that it can be "too late" to change transmission fluid is based on the potential that so much varnish and sludge has built up in the transmission that the detergency of new fluid will soften, loosen and break up this material and cause it to clog hydraulic passages, potentially leading to transmission failure.

Can this happen? Yes, just as your service provider described. But at this point you have limited options. First, I'd suggest adding half a can of SeaFoam Trans-Tune to the transmission fluid. The solvent action of the Trans-Tune can help dissolve varnishing to help improve hydraulic flow and pressure. Second, as I described above, you can have the transmission flushed and refilled with a new filter. Or third, you could do nothing and hope for the best, which, in my humble opinion, would not be a good choice.

One final note. It is entirely possible that high ambient and high operating temperatures contributed to the slippage you experienced. Fresh transmission fluid would be less sensitive to these temperatures.