So basically, unless I have seen many analysis reports for a given engine, and I have correlated these results with tear-down observations, it is hopeless for a novice to try and determine these things?
In terms of oil analysis data, I'd use these limits:
Total Solids < 0.7%
The variable that most effects TS levels is average trip length and speed. Short trips at low speeds lead to sludging - particularly in cold temps ....
TBN at or above 2.0, w/ ASTM D-4739
TBN at or above 4.0, w/ ASTM D-2896
You want active detergency/dispersancy at the end of the change interval ...
Viscosity Increase < 25%
Tied to oxidation/nitration levels and the # of partially burned fuel particles. Tied to soot levels in diesels ....
Oxidation/Nitration < 50% of their "allowable" limits.
Note: Many labs report Ox/Nit in "abs/cm" and typically use 30 units as the limit for petroleum lubes and 50 units the limit for synthetics. So you have to take the raw data and double it to get a percentage of the max allowable ....
If so, what do you look for in the results that would indicate the engine is getting dirty?
There is probably no "clean" answer that the pros can provide without giving away trade secrets, but I figured I'd ask anyway.
TBN depletion rate and level
Nitration increase, oxidation increase
wear signatures specific to engine design increasing.
TAN increase rate and level
insolubles increase rate and level
condensation rate and level
flash depletion rate and level
to name a few.
Keep in mind variability exists thus the difficulty in each engine and report discerning what the TOTAL result means.