Synthetic ATF handles a lot more heat before it breaks down. The Allison 6 speed in my big truck specifies 15k intervals with dino ATF and 75k with synthetic ATF. At 75k you sample it and replace if needed otherwise you go another 25k and sample again. I'm not going to go 75k on my car but it's right at 50k (due for a change) with synthetic and still looks great unlike dino ATF at 25k.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k