When you say 4.2 V8 are you talking about the M119 DOHC engine from the W124 and W140? (400E, E420, 420SEL) If so they run a bit warmer than the 6's in the W124's for sure. I don't have as much experience with them in the W140's but in the W124's the car will run 80-85 cruising, and about 100 C sitting in traffic with the A/C on. If the temp gets above 110 C at any time or stays at 100 C while driving at highway speeds you may have a problem. If it just creeps up a little while idling in traffic that is normal. You should hear the electric fans come on when it gets a bit over 100C. The fan clutch should also engage at about the same speed.