Wouldn't another avenue for fuel in your oil be piston ring blowby? The Diesel engine oil seems a logical approach, as typically the oil in a D. engine becomes quite runny with fuel admixture and in a very short run time. A bleed down test on each cylinder might be revealing. If one cylinder tested out noticeably weaker than the other, a hone job or a new sleeve with new rings may solve your problem. My two and a half cents, cheers.
