VolvoNV,

I very much doubt that a 1/2 quart of trans fluid made any difference in mileage.

As someone who has recorded gas fillups in a log book, for an entire life of several cars, I can tell you that variations of +/- two, or three, MPG, between any two fillups, for like driving conditions, are not uncommon.

Every gas pump "clicks" off at a different point. If you filled up your gas with the car pointing downhill, or uphill, you will get dramtically different results. A single fillup, even two fillups, cannot give you an accurate mileage picture.

Think about it. If a 1/2 quart of trans fluid made a 15% difference in mileage, then your trans was dragging your car down and it would have felt as if you were pulling a trailer. Did it? That much drag would have elevated your transmission temperatures to the point of damage.

Point is that no manufacturer designs their transmissions to self-destruct when they are down a half-quart.

The fact that your mileage "improved" after topping up your transmission was a coincidence.

As to "ATF was meant to be a lifetime fluid/you'll oxidize it by opening the dipstick tube". This implies that the transmission system is hermetically sealed. It is not.

Hope your mileage continues to improve.