As far as what's on the magnet, if that's the first change ever or the first change after a rebuild from way in the past, don't worry about it, that's a normal amount. Just shows you the magnet is doing its job. Ditto if it's been a long run on the oil. What oil there is in the pic looks very brown, however, which leads me to believe it's been a long run. Or a very hard one.
Eleftherakis & Kahlil, two engineers who literally wrote the book on auto trans contamination thru decades of research, found that the average automatic generates 75 percent of it's lifetime amount of contaminants in the first 5K miles of use, either after manufacture or after a rebuild. That's the junk built in from manufacturing/rebuilding and from break in. The amount built in is variable according to how good or bad the manufacturing/rebuilding process was. The Japanese and Europeans have been better at clean AT manufacturing over the past 25 years or so, but the Yanks have caught up lately... by necessity, I think. Break in is still an issue. The indicator is to either a) change the trans oil very early, in that 5K range, or b) add an external trans filter right away to clear out all that contamination.
After break in, the rate of contamination levels off at a greatly reduced rate and the contamination level in the oil increases at a steady rate from normal wear. Contamination rates are often related to the use of the vehicle. Easy use = low rate and vice versa.
The main goal then is to monitor fluid condition and change the oil when either, a) the contamination level rises past about 25 mg/l or, b) the fluid has oxidized or is depleted in some way. In most cases, without additional filtration, the contamination level rises past an optimal level before the fluid is depleted (this is an across-the-board, generic statement; there are individual exceptions).
For long term service of the oil, some addition trans filtration (beyond the pad in the pan), which will hold the contamination in check so the fluid can live a natural life. If the contamination level is maintained below 25mg/l (the optimal level is below 10 mg/l, all this was determined by Eleftherakis and Kahlil) then in an easily used trans with a premium fluid in which the temps have been kept below 230F, ATF life can indeed be virtually considered "lifetime" in many cases. Certainly 100K +.
Determining all these conditions does require extra effort on the part of the owner. For the most part, IMO, if you have a good supplemental filtration system (an inline filter is fine), you could pretty much tell by fluid color but I am pedantic enough to want to do a periodic UOA (every 30K or so) long enough to determine some working guidelines as to the life of the oil. Once I have a basic pattern, then monitoring is unnecessary IMO, until some part of the equation changes (operationally or the oil).
Now as to what trackspeeder said:
Originally Posted by trackspeeder
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The only thing a deeper pan does is cool your wallet of $$$.
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You are basically right, though there are advantages to more fluid. Unless the deeper pan has cooling fins or tubes, the deep pan in and of itself has minimal effect on
stabilized pan oil temp. Because a deep pan has more surface area, there is a little more cooling effect just from that, but it's not enough to argue over unless the pan had fins. The things a deeper pan DOES do is slow the rate at which oil temp increases. It takes longer to heat up an 8 quart volume of oil than it does a 4 quart volume, but over time the greater oil volume will still reach more or less the same operational temps as dictated by the operating conditions as the lesser volume. The deep pan also allows for an extension of service life because of the increased oil volume. A trans generates "X" amount of contamination per 1000 miles no matter what and if it's diluted by more oil, you can run that much longer before you have to change oil based on the mg/l contamination levels in the oil
