Efficiency at full rated load is maximum or could said under loading or overload(short time) would be at the cost of efficiency or extra life consuming cost of DEGs, could refer my paper …..

Need to consider checking the air cleaner, could be fuel is not clean so it is not fully burnt, or even the seals which allow oils into the chamber yet mixing with fuel and also the cooling system, sometimes if the cooling system is having any issues then it makes the block to expand hence producing holes that let oils and water into the chamber, rendering the engine inefficient