I don't have that much to add, I do like the 2000-2001 C-15 engine (6NZ= Serial number Prefix).

It isn't a perfect engine, but it is around 180 lbs lighter than 3406E, has huge improvements in the oil seepage/leakage problems at the front and rear cover areas(over the 3406E) In Fact; I have never seen a rear cover(flywheel housing/bellhousing) oil leak on these. [This was a big deal, to any CAT mechanic, that was sooooo tried of repairing oil leaks/seepages on 3406E]

Now, the C-15 didn't seen to hold head gaskets, as well as the 3406E, but CAT has came out with new and improved head gaskets for this issue. So far, so good, but those gaskets probably don't have 500,000-700,000 miles on them yet, thus the verdict is still out.[NOTE: the 3406E models did really well on head gaskets, in my opinion]

Why the difference? IMO: CAT started machining expansion cuts, between cylinders, on the cylinder head "bottom deck" of a C-15 engine, thus the gasket lost some clamping force in those areas(in-between-cylinders/liners)

But I'll take a C-15(6NZ) anyday, and If you don't mind some oil seepage, than the 3406E "1LW" and "2WS" prefixes have been really good. (I just don't like oil seepage/leaks!!!)