Why not use the right stuff for the forks, instead of experimenting with ATF, motoroil (some use that...), etc..? Yes, it cost a little, but how often do you really need to replace it...
The right stuff always was ATF, before fork oil came along.
Which fork oil should I use ? A 7.5W of one brand has the same viscosity as a 10W rated other brand, and even a 12.5W fork oil can have the same viscosity as another brand 10W, or even 7.5W.....which leads me to conclude one thing - if there is no standardisation of viscosity, relative to the weight rating, and considering that fork oil performance is directly related to viscosity, not 'Weight' rating, then what we need is an oil that is seal friendly, designed for pressurised hydraulic systems, has low moisture absorption, wide temperature range,and no matter which manufacturer you buy it from it has much tighter viscosity ranges than any fork oil. For example Dexron 2 has an almost identical viscosity across all manufacturers, because it is rated and specified , properly, in terms of viscosity.
You know where you are with ATF, it is a safe to use, standardised viscosity, with a predictable performance range, which is useful.
Here's a table of different viscosities of fork oils - https://s3.amazonaws.com/advrider-photobucket-images/images/J/JakeSabre1_ForkOilCap1.jpg Do you know which one you need ? Do you buy it by weight rating 'W' or buy lab tested viscosity ? Do you know that one manaufacturer's light oil is the same viscosity as another manufacturers mid weight oil ?
Dexron 2 has a viscosity of 37.5 at 40 celsius, and thins predictably with temperature. Meeting the standards set out for General Motors Dexron II, Mercedes Benz and Detroit Allison C-3. In fork use, it is of about the same viscosity as Bel-Ray 10W. Possibly a bit too high for cartridge forks, but for TDM forks it's absolutely fine.