[Vwdiesel] oil change intevals...

[James Hansen]

Matthew:
If you have to know exactly, you should get an oil analysis.  Any
agricultural or industrial shop would be able to point you in the right
direction for that, you buy the kit, and send it away in it's mailer to
their lab.

I hope metal shavings aren't your main reason for changing the oil :-)

 What is the big deal in a diesel, is the soot.  Soot isn't a lubricant, and
diesel oil has soot handling additives, so that it stays in suspension and
doesn't settle out, which is the main difference between diesel and gas
engine oil.  More thorough filtration is an effort to get some of this soot
out of the oil, and allow it so be used longer, and actually wear the oil
out, rather than change it because of contaminants.  Soot also changes the
viscosity performance of the oil as the loading increases, and is a big deal
if you live where there are seasons.  There is no hard and fast rule about
intervals, it's pretty much engine specific, depending on the usage.  Things
like highway miles vs city miles, turbo vs NA, overfueled vs underfueled,
good intake air filtration vs poor,and load on the engine,  all have a
bearing on change intervals.
You really only know with an oil analysis. Once you have established this,
then you can use up your expensive synthetic, rather than throwing it out
when it's maybe only a quarter used.  I know in my situation, the seasonal
requirements dictate oil changes for me, and it is well within the lifespan
of the oil, but it has to be changed to a lighter grade, or not start in
winter. 10-40 synth blend in summer, 0-40 synth in winter (Shell).

The bit about the engine by-products being lubricants in a diesel, is an
urban myth methinks- other wise you would never have to change oil. It's
more a factor of what isn't there, the nasties from a gasser and gas itself.
Carbon chunks sure aren't lubricants, and do accelerate wear.  Diesel fuel
that gets into the oil (rare in a properly functioning diesel) is oily, but
not really a lubricant either... but better than gasoline for sure, and it
is quite common for gas to get into the oil on starts in a gasser. I wonder
if this line of thinking doesn't come from people knowing that graphite is
carbon so assuming it has the same properties as the fine particulate carbon
in the oil. If that was the case, you would have to periodically pull the
pan to remove the pesky diamonds that have accumulated there... but we know
carbon doesn't change from a low energy bond state to a high energy state
without a lot of encouragement.

You do still change filters when running high mile drain intervals, so don't
overlook that.  Gotta keep the diamonds from getting into the bearings. ;-)
-james



-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Matthew
Sent: Friday, January 20, 2006 10:21 AM
To: diesel fans; audi-vw-diesels at yahoogroups.com;
tdi-conversion at yahoogroups.com
Subject: [Vwdiesel] oil change intevals...


Folks,
yes, i know 3K miles for oil changes is a good habit
and one i practice. But...

we change oil on a gas engine because of acid and gas
buildup in the oil (and minute metal shavings, of
course). But on a diesel, my understanding is the
diesel fuel + combustion prioducts are in fact
lubricants themselves, and don't foul the oil like gas
engines. So, other than the minute metal shavings, why
do we change out oil every 3K miles, when we could
probably go much longer? Assuming we have not cooked
teh oil from running hot and are running top notch
synthetic (rotella, amsoil, etc).

thanks
matthew
whose rig needs an oil change (and will get one tomorrow...)

"Racing with the wind and flirting with death
So have a cup of coffee and catch your breath"

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