I have some questions on changing the transmission fluid on a 98 ML320 with 95K.

I found the trans fluid fill hole and it is covered with a black plastic cap with a red tab on it. How do you remove this cap? Is it supposed to be broken off? Fastlane sells a $50 dipstick that says it's for diagnostic purposes only. Can the dipstick be left in there, or must it be replaced by the tube pin?

The fluid is $14.60 per liter. Must this fluid be used, or are there competitive products that are cheaper? How much should I buy to change the fluid in the pan and torque converter?

Is the pan gasket reuseable?

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95 E320 Cabriolet, 159K