I hope someone can give me some ideas what is the problem with my automatic gearbox

I bought a IS200 -99 with auto gearbox. When I start the car when it's cold and put it to D... The car seems to slip/rev a bit too much and there is a small delay before the car starts moving when I push the gas pedal. The hesitating is something like less than a second but I noticed it immediately because it is noticeable enough to feel kinda very "weird". Also when cold the kickdown doesn't seem to work when I mash the pedal. The car is changing gears without a problem when cold and there is also no delay between changing from P to D etc... Also I don't notice any powerloss or slipping when driving cold or warm.

When the car warms up it's 100% fine and nothing wrong with kickdown or that kinda "slipping" in low speeds. Repair manual says it has had gearbox fluid changed in a toyota shop about once per year/10000km for the last 4 years so there seems to be something wrong. Also last change was 8000km ago and fluid is still almost blackish and a bit burnt smell

I was thinking maybe that small toyota shop put wrong oil in the box few years ago because there was a castrol sticker next to that repair notice. I think these auto boxes are the same as supra transmission that they require the toyota type-T oil to work properly?

Do you think there's any chance that it might be fixed with a flush and fluid change or better to buy a new second hand transmission straight away? In here you can only buy Toyota type-T fluid in 5L jugs so not that cheap of a trick to try if the box is broken