I honestly don't know much about the 617 clutches. I don't think they leak, they seem to be a different design. The 603 clutches are always wet after a few months, and they collect dust and grime all over the face. You clean it off, and it's back a few months later. I've never seen that on my 617 clutch. Also the 617 clutch is easy to check with the "roar" test, plus it stops instantly if you kill the engine with the clutch engaged on a hot day. Not so on the 603, never roars, and just stops slower - never seems to fully lock. At least with the newer 606 clutch (which both of mine have). Kinda odd, and makes it hard to test!!
On a related note, my 617.952 runs at incredibly steady temps - 85-90C almost all the time. Even in brutal summer heat, whether idling or climbing mountains, I can't get it over 95C. Very, very stable. To me that indicates an excess of cooling capacity with things in working order (I have recent radiator, clutch, t-stat, etc). The OM603, even when perfect, will run hotter. High speed runs using max power will drive the temps to 100-105C even on level ground with cool ambient temps. That's the price you pay for 20% more horsepower with basically no increase in radiator area (I'm assuming). It shouldn't run towards the red (110+) unless something is wrong though, and it should cool down within minutes after the load is removed. At least my 603's behave that way!
