A high-output Army two-cycle diesel engine was subjected to proving-ground related laboratory-dynamometer endurance test cycles using four different qualified MIL-L-2104C SAE grade 30 lubricants. Three diesel fuels differing mainly in sulfur level and end point temperature were an important part of the test matrix. Two lubricants (low- and mid-ash level) produced acceptable performance and a third (of high-ash level) was considered borderline acceptable--all during operation with reference No. 2 diesel fuel (0.42% wt sulfur). A fourth oil (low-ash level) was judged to be incompatible due to its proneness to severe piston and liner scuffing. The engine was judged incompatible with a high sulfur/high end-point [1.2%S/396°C (744°F) EP] fuel intended to meet MIL-F-16884F (Marine Diesel Fuel) using two different MIL-L-2104C lubricants. This engine was also judged to be incompatible with a special blend of NATO F-54 diesel fuel (0.64% sulfur) during operation with the same two lubricants. Incompatibility using fuel sulfur levels greater than 0.50% was based on the occurrence of catastrophic piston/ring/exhaust valve failure and relatively high deposit and wear levels.