Importance of oil changes

[stefang]

Hi all

Yes, I know they're important, but specifically I'm looking at an '86 with 105k on the clock that's only been driven 4k miles in the last 9 years. Oil changes were sporadic at best. How bad is that? Let's assume that the compression/leakdown come back good. What could go wrong?
I guess the quantifyable downside is an early rebuild. Anyway, The car looks good apart from that, so I'm debating...

Thanks for all opinions.

Stef

[1fastredsc]

If it has only been driven 4k miles in the past 9 years that would explain the sporadic changes. Most oil manufacterers (or at least the synthetics) recommend like 3k/5k miles or X amount of months, whichever comes first.

[stefang]

Well the x months in this case seem to be about 24 or so. On mileage alone he really didn't need any in the last 9 years.
It's just that any contamination had longer time to work on engine parts. OTOH - Fresh oil ceases to be fresh once you run the engine, so you always have the same contamination in there anyhow.

Stef

[targa911S]

105 K on that engine is just broken in! Have it checked for comp/leak, adjust the valves, plugs, wires, change the oil in engine and gearbox and have fun.

[Bill Verburg]

Oil doesn't wear out. The reason it does need to be changed regularly is to remove corrosive condensates from the engine. An engine that is never used won't need an oil change but that isn't good either as the seals dryout and take a set which will then leak. If it isn't used much but oil is changed sporadicaly that is likely better than not being used at all.

When I put mine a way for the winter it is always w/ fresh fluids. If is started during the off season the engine is allowed to get thouroughly hot before being put back to bed.

[moazam]

Originally posted by targa911S
105 K on that engine is just broken in!

That makes me feel so much better about my 173k miles '86 Coupe.

[Jim Smolka]

Check the head studs and then the Cam for wear.
If good (Mahle) filters have been used, things maybe OK

[dickster]

Oil doesn't wear out. The reason it does need to be changed regularly is to remove corrosive condensates from the engine

bill,

it does breakdown though?? at high temps etc. it also gets contaminated with carbons which is why it turns dark in colour - thats the time to change it.

i wonder if it is like brake fluid and absorbs moisture over time?

[Bill Verburg]

it does breakdown though?? at high temps etc. it also gets contaminated with carbons which is why it turns dark in colour - thats the time to change it.

At high temps yes, it does brake down. That doesn't appear to be the case here though.
Contaminated yes, if the rings are in poor shape, a normal motor w/ such low milage isn't an issue.
Oil is not hygroscopic like brake fliuid. It does not absorb water. But water does condense internally and mix w/ the blow by products to produce the corrosive mixture which causes the damage. Every thermal cycle cause additional coondensation. In normal use a large portion of the condensate is boiled off. In winter and short trip use the condensate does not get boiled off, it accumulates

[Randy Webb]

"Oil doesn't wear out."

- But the buffers can get overloaded after enough combustion acids are buffered.

- BTW, the condensate can appear as a brown whipped cream looking "sludge" on the cap.

- Also, various chemical components can break down at different rates and that will affect lubricitiy and other functions.

[Bill Verburg]

YEs, all modern automotive oil has detergent packages that will keep the contaminants in check to a point. The detergent packages are one of the major differences in oil brands and types. In this respect synthetic oil is generally far suprerior than dino.

If on the other hand you are using straight mineral oil(aka 2 stroke oil) there are no detergent packages to help keep internal corrosion at bay.

[Randy Webb]

Bill, I wouldn't use the term detergents -- the additive packages contain detergents, buffers, lubricity enhancers and more. The term detergents is too specific.

I don't mean to be anal -- it's just the chemist in me.... or remnants therof.

Back to the guys query -- best would be to examine whether the car was used on short trips or not. If no data, then consider that a risk factor and bid accordingly -- almost any 911 has some value...

[stefang]

Sweet - haven't started an oil flame war since my ducati mailing list days
This is a pretty good write up for all us non chemists: Motor Oil FAQ

From his description, when he did take the car out it was for longer trips, so that's a plus.

Is checking cam/headstuds part of a "standard" PPI? I just finished reading a thread about headstuds busting in the first week of ownership, so I'm a bit weary. I wasn't really planning to buy Wayne's latest book quite yet.

Stef

[jess5engine]

Funny this should come up. I was reading Bruce Anderson's book last night and I quote:

"There are other conditions that can cause the top-end life to vary, such as cars that are driven infrequently. When the engines sits idle, the moisture and acids will corrode the engine's internal components. The only way to avoid this corrosion is to drive the car frequently so the engine heat will dry out the moisture. Frequent oil changes will also prolong the life of an engine that is not used as often as it should be. Regular operation also helps keep the rings free and will help avoid carbon buildupon the piston and combustion chamber. When an engine sits, the moisture will soften the carbon buildup and pieces of carbon can get caught between the valve and the valve seat, which can cause a burnt valve."

Just something to consider when you get a PPI. Good luck.

[pwd72s]

Originally posted by jess5engine
[B]Funny this should come up. I was reading Bruce Anderson's book last night and I quote:

[1fastredsc]

I thought that over time if oil just sits it combines with contaminents and creates a corrosive acid?
EDIT: Apearantly i'm too late since i just read the previous posts.

[Bill Verburg]

I don't mean to be anal

Too late

I wouldn't use the term detergents

Fine but thats the industry argot, just 3 examples to add to stefangs

oil1
oil2
oil3

there are a many more

The discussion was on a limited use engine and potential consequences, not lubricity, anti foam et.al.

[Randy Webb]

Bill, you understand -- of course -- that in this context, anal meant analytical -- as in analytical chemistry (as opposed to synthetic chemistry)?

I'm sure you didn't take it biologically...

[ted]

Question.
How often would you suggest I change oil on my track car.
In this case....
I use Castrol 20/40
I see 300 miles on a typical weekend of track driving.
One event each month.
Oil temp is always 210 or less.
Shift at 6800rpm.


How about gear oil on the 915 trans, no trans cooler.
I use Swepco red.
Is the red Swepco required or would the cheaper Swepco work as well?
thanks
Mike

[dickster]

mike

if i were tracking regular i would change after each meet. 300 track miles are the equivalent of many more road miles.