On my starboard engine we removed the 3- 86 deg C thermostats. The rubber on each of the thermostats is destroyed. The rubber looks like it was eaten up. No problem on the thermostats on the port engine. Both engines have the Man approved coolant in them. When we were removing the heat exchanger on the starboard engine we found alot of rubber pieces but didn't know where it was coming from until we removed the thermostats. No one from Man has been able to explain why this has happened.