Three aged gas-oil based motor oils were prepd. by addn. of (i) ethylhexanol soln. of Na ethylhexanoate (Na content 0.5 ppm) and dodecylsuccinic acid (70 ppm), or (ii) poly-isobutylenesuccinimide PIBSI (120 ppm), hydrogenated di(oleic acid) (100 ppm) and HCOOH, or (iii) fatty acid Me ester (10% by vol.), 2-ethylhexyl nitrate (500 ppm), and PIBSI (120 ppm) and then combusted in Diesel engine to study both external coke deposits and internal carboxylic acid ones on fuel injector and nozzle surfaces. The Fourier transform IR study showed the presence of products of additive oxidn. and amide structures.