Is it warm? The oil temperature should probably be at least 80
degrees C (the Bentley manual would specify this). Hydrocarbons can
build up in the exhaust also. Drive the car in third or fourth gear
for several miles on the highway and then don't shut the engine off
until after the test. This will warm up the engine and blow out the
exhaust. If this fails, change the oil. If that was just done or if
that also fails, then you will need to look deeper.