I wonder if 90 degrees is even hot, though,
If you think about it, with an 87 degree thermostat, the temp that your t-stat starts to open is 87 degrees - not the temp that it is fully open, but the temp that it starts to open. I'm sure your car should be running between 87 and 92 degrees most of the time when running on the freeway, and will probably run about 100 degrees at an idle in hot weather. Here is what the CD says about the cooling system and temperatures on M104 engines:
Thermostat will start opening at 85 to 89
Thermostat will fully open at 102
Viscous fan clutch will cut-in at 96 to 104
Viscous fan safety cut-off will ocurr at engine rpm 4500
The same will ocurr at fan rpm 3250
Electronic fan (1st stage) will cut in at 16 bars of refrigerant pressure)
Electronic fan (1st stage) will cut out at 11 bars
Electronic fan (2nd stage) will cut-in at 107
Electronic fan (2nd stage) will cut-out at 100
A/C will go into 50%on, 50%off cycle (20 seconds interval) at coolant temp 121 to 123
A/C will go into emergency shut-off mode at 126 to 128
Pressure cap will open at bar 1.3 to 1.5
Coolant temp max is 130
Remember that water boils at 100 degrees celsius, but with a 50 / 50 mixture of water to coolant, your boiling point is raised to 120 degrees, and the car isn't considered to be overheating until the temp exceeds 130 degrees.
MB designed these cars to run hot, and 100 degrees is not even considered close to being hot. Your thermostat isn't even in the fully open position until 102 degrees. Further, the 2nd stage high speed fans comes on at about 107, and shuts off at 100! How can 100 be considered hot if that is the temp that the high speed fan shuts off at?
Now, many of us on this forum (including me) doubt the wisdom of the brilliant German engineers for these cars, and carry out modifications to prevent the car from going much over 100 degrees.
I have installed the "cool harness" that JimF sells, and it causes the high speed auxillary fan to engage at 100 degrees, and runs until the temp reaches 82 degrees.
I am convinced that your car is running at exactly the temp it is supposed to, but I'd still only run original MB coolant.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".