I have a 66 3.8 s-type & it rund just under 90degrees on the gauge.
The temperature the engine runs at is also affected by tuning. When I first had hte car it ran just above 70degrees but didn't run well. Having had the car tuned on a rolling road by Tom Airey, the car runs faultlessly but hotter. The car does drive totally diferently now, loads of power & very responsive. The temperature will rise 4 or 5 degrees if sat in traffic on a hot day but no more than that.
Hope this helps, Jacko