1997 E320. Please tell me where should the power steering fluid level be on the gauge ? max at 80 Celsius or max at 20 Celsius?

That depends on the temp of the fluid. If you have been driving around for a long time and the fluid is hot, then go by max at 80C.

If the car hasn't been driven and the fluid is cold, go by 20C.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record