Lightman

[quote author=1sttruck link=board=20;threadid=23603;start=75#msg227723 date=1072112388]
Regardless of how stable the TBN, viscosity or such is with a synthetic it still won't reduce the soot level in the oil, so the change interval can't be increased.[/quote]

Incorrect sir! That would be true if soot were the limiting factors in our oil changes, however it's not. Our HPCR engines burn so cleanly, that soot is always very very low. The determining factors for oil drain on our vehicles are wear metals and tbn. Therefore extended drains are possible with quality synthetics.

1sttruck

If wear metals and TBN are the primary factors for determining oil change intervals, why does the 2003 CARB SO engine have a 3750 severe duty oil change interval and the 49 state SO and HO have a 7500 mile severe duty oil change interval ? I wouldn't think that it's wear metals, but it could be TBN. I think that the 2002 and earlier engines also had a 3750 severe duty oil change interval. I'll still guess soot based upon a reply from another newsgroup on the CARB and non-CARB engine differences:

Posted by: ACerf

THIS IS THE STATEMENT I RECEIVED FROM POWERMASTERS ON THE SUBJECT.

Summary: ram isb CARB ratings by year

Solution:

Thanks for the email. The primary difference is more retarded timing, which reduces engine efficiency but does lower peak temperature for lower NOx. This also causes increased soot loading of the oil, as the fuel gets quenched by cylinder walls and produces more soot in the oil.

A lot of the exhaust energy is absorbed by the turbo; the increased back-pressure from the converter is not likely much of a penalty

Lightman

oh well you could be right with carb engines, i'm really not sure, however I'd be surprised if they were THAT much sootier. The oil analysis reports I've read have been for non carb 3rd gens...but they had almost no soot at all.. Some experts say max allowable soot is 6%, some say 3%, opinions vary, however the cummins oil analyses I've seen, the soot is like 0.1%, 0.7%, stuff like that, not even close. I'd be interested to see a carb oil analysis to see how they differ.

bironside

Not to try and keep it going but here is the petro-can info
http://www.petro-canada.ca/eng/prods...eases/9168.htm