When you checked the auxiliary fans, did you check to make sure they were turning in the right direction? I know it sounds stupid of me to say that, but I have heard of more than one occassion when the auxiliary fans were removed for some reason, and when they were replaced the connector plug is put in backwards, resulting in the fans turning backwards. Your fans should pull cold air in from the outside across the radiator. If they are reversed they will be pulling hot engine bay air across the radiator, which will only heat the car up more. Were the fans removed when you replaced the viscous clutch?

If the fans are going in the right direction, I would look at the thermostat. It might not be opening fully so you aren't getting proper circulation, which would lead to the car heating up when it is under more load (ie on the highway or going uphill). It could also be your radiator going bad, which wouldn't be out of the ordinary for a car of the W202's age.

I don't think this has anything to do with your choice of oil, unless the oil has been dramatically underfilled.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)