This is a drain plug O-ring for the 290-DP, DP-E other DP drives listed below. This is the larger option for an MS25S and some 120S-E Saildrives whereas the other option, part number 3855081 is a much smaller O-ring measuring about 6-7mm.
Volvo Penta green coolant gives vastly superior corrosion protection with special additives for aluminium protection. Suitable for both petrol and diesel engines. It also has good anti-freeze properties