Dave R.

My U.S. owner's manual specifies an oil interval of 15,000 miles. That's a loooong interval.

Some of you long interval (10k-15k miles) RoW guys cheerfully give us short interval (3k-6k) US guys grief for throwing good oil away, and we reply in essence "I do it because it makes me feel good."

There is a good technical reason to do a short interval, though.

Bear with me while I briefly set up the arguments.

I see three basic reasons to change the oil:

1. The oil is losing its lubricant properties (thermal breakdown, etc.).
2. The additive package in the oil is worn out (all the stuff that neutralizes acids, holds soot in suspension, etc.).
3. The oil gets dirty with particles too small for the oil filter to cleanse.

With a good synthetic oil, I think #1 is generally not a significant issue out to 15k.

#2 depends on driving habits, short trips can clobber even a synthetic oil's additive package well before 15k. (See for example this <a href="http://www.practicingoilanalysis.com/article_detail.asp?articleid=348&relatedbookgroup=OilAnalysis" target="_blank">article1</a> which also describes a clever "business card" oil test).

#3 justifies a short interval. The full-flow spin-on oil filters we use don't effectively capture particles less than 10 microns across. As it turns out, many of our lubricant film thicknesses are less than 10 microns (e.g., piston ring - cylinder wall lubricant film thickness of 3-7 microns), which means that these smaller particles cause wear and power loss. (See this <a href="http://www.practicingoilanalysis.com/article_detail.asp?articleid=401&relatedbookgroup=PowerGen" target="_blank">article2</a> for a nice explanation with additional info).

The only way to remove those sub-10 micron particles is to change the oil. That is a sound (even excellent) technical justification for a short oil change interval, I think.

Note, the second article I cited above mentions a study in which filtering out the small particles reduced the engine wear rate (as measured by oil analysis) by 90%.

Final note: Amsoil sells an auxiliary bypass filter that purportedly cleans out particles down to 1 micron or smaller, see this <a href="http://www.amsoil.com/products/bf.html" target="_blank">link</a> for details. Looks nifty.

So, comments? <img border="0" alt="[bigbye]" title="" src="graemlins/xyxwave.gif" />

Ruairidh

Interesting and helps me understand the differences in common practise and prevailing wisdom re: oil changes in the US and outside. I cannot remember ever changing my oil in the UK as often as I do here with oil changes at the 3k or 6 month level. Mind you the price of oil is so much cheaper in the US which may have a serious impact on the cost/benefit analysis...

914und993

I'll bet this has been hashed and re-hashed to death on this forum already, but hey - I'll bite.

Keep in mind that the 911 oil capacity is about 2.5 times greater than the average automobile. That suggests that if wear rates in a 911 are similar to other vehicles, you would have to drive about 2.5x more miles to get the same concentration of wear particles in the oil as the average car. So if an 'average' car can go 6000 miles between changes, a 911 could go 15,000 miles before reaching the same point.

I also have a nagging suspicion that Porsche has actually looked at this in an empirical fashion, and conservatively decided that their engines can go 15,000 miles between oil changes without incurring accelerated rates of wear.

Now I'm not trying to tell you not to change your oil every 3,000 miles. If it makes you feel good, then do it since it won't hurt (excepting the accelerated wear on the threads of the drain holes, heh heh). //censured politico-economic joke goes here// Just kidding....

Chip

Adrian

If you use mineral oil you should change it every 3000 miles or 5000 km. If you use Synthetic oil 12,500 miles or 20,000 km or 2 years whichever comes first is the normal European limit. 15,000 miles is recommended in the USA with synthetic oil.
These intervals are for "normal use". If you fall into the "Severe use" category then you have to reduce the interval. Severe use also includes low mileage. Low mileageis regarded as less than 5000 miles or 8000 km per year. If you are at this usage you shoudl change your oil every year regardless of its type.
Please remember that these are just recommendations or guidelines. Once the 964 is out of warranty you can basically do whatever you are comfortable with.

One last point and I put my moderators hat on now. No politics on this forum thanks kidding or not.

Ciao,
Adrian
911C4

PS: For our middle east Rennlisters driving in the desert atmosphere is also regarded as severe usage.

Roygarth

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by 914und993:
<strong>

//censured politico-economic joke goes here// Just kidding....

Chip</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I bet this was a reference to how we should delay our oil changes until after Bush has taken Iraq, to ensure our dollars flow into his friends pockets rather than Saddams?!

Dave R.

I appreciate the comments.

Yes, 911s do have a large oil capacity, I agree that is definitely a good thing in all respects.

Dino vs. synthetic (#1 of my post) and additive package life depending on type/frequency of driving (#2 of my post) have both been hashed out on this forum and the 993 forum.

However, I've seen no discussion of #3 (dirt particles 10 microns or smaller that are too small for conventional filters to trap). The closest is Mapes' post at the end of this <a href="http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic;f=3;t=006191" target="_blank">thread</a> pointing out that a partially loaded filter is actually a bit more efficient than a brand new filter.

Regarding "acceptable wear rate", I suspect Porsche AG's idea of "acceptable wear rate" is different than mine. They may think that an engine rebuild at 150k or 200k is an acceptable wear rate; I'd prefer 300k or 400k. Also, Porsche considers other things besides engine life when they set the oil drain intervals. For example, oil changes cost money, and since cost of maintenance is a marketing issue they have an incentive to lengthen the drain interval. Also, Porsche is a target for environmentalists, who dislike production of both new and used oil, so that is another political/marketing incentive for them to set a long drain interval.

The observation that Porsche qualifies driving in a desert atmosphere as "severe usage" is interesting. My bet is that dust and sand in the air are the reason, rather than high temperatures.

I think the difference between official US and ROW intervals (15k miles vs. 20k kilometers/12.5k miles) is a matter of custom more than anything else. Here in the US, manufacturer-specified service intervals for most cars are typically in multiples of 15k miles (7.5k for oil changes), in Europe in multiples of 20k kilometers (perhaps 10k for oil changes). People are used to seeing these numbers, and the intervals are sufficient for most purposes.

Regarding mineral oil at 3,000 miles, my 1990 owners manual Porsche specifies a normal drain interval of 15,000 miles for mineral (dino) and synthetic alike. The manual indicates that synthetics can be used over a broader temperature range than mineral/dino oils, but the drain interval is the same.

There is nothing wrong with a statement like "If you use mineral oil you should change it every 3000 miles or 5000 km." However, it adds little to this thread because it does not indicate WHY mineral oil should be changed at 3,000 miles instead of 15,000 miles like Porsche originally recommended.

914und993

I would certainly concede that Porsche may have external pressures to state a longer oil drain interval than perhaps ideal, but I'm not so sure myself that this has happened.

Although I have never left oil in my Porsches for 15,000 miles, there are circumstances where I would have absolutely no qualms about doing so. If I put 300 highway miles per week on my car, at moderate speeds, and it had a nice fill of Mobile 1, and I had no days at the track, and encountered little in the way of dust in my driving, then I don't think it would bother me a bit. I have used oil analysis off-and-on with various cars, and the only deviation from 'oil still good' that I have seen in the 911 is an increase in the viscosity over time beyond the range expected for the oil that was used to fill and top up. And this was with synthetics, too. As a result, I now usually top-up with a lower viscosity oil than used in the original fill.

As for the particles less than 10 microns in size, I'm really in no position to comment on their effect on engine longevity. It would be interesting to see a study by an impartial party on the rate of accumulation of such particles in a gasoline engine, and the actual effect on wear items in that engine. All the 'information' on that subject I have come across has been from 'tests' by parties with a financial interest in a particular outcome. And of course there is plenty of speculative information on the subject.

As for the difference between USA and RoW, one author of a 911 buying guide strongly recommended against buying a Euro 911 of any significant mileage, since he felt the cars were driven much harder over there. Perhaps that is true. There isn't much opportunity here to tear along at 100+ mph for extended periods like might be possible in Germany. It might also be that for a given number of miles, a US car has fewer cold starts since distances between points tend to be greater in the USA. All of these might have an effect on engine oil longevity.

In the end, if you love your car, changing more often rather than less often is probably the safest route, but it might not actually be time or cost effective in an absolute sense to do so more often than Porsche recommends.

Chip

Dave R.

Chip,

Interesting idea, topping up with a lower viscosity oil.

I think following Porsche's recommendations will result in satisfactory service life for most people. I also think that shorter drain intervals will extend the time between engine overhauls. I think we agree that different folks will decide differently what is cost effective for them, and balance the tradeoffs in different ways. And that's OK; I don't think everyone must or should go to a short(er) drain interval.

As to particles, two questions for you.

Did those oil tests you purchased report concentrations of 1-10 micron particles, 10-20 micron particles, etc.?

Did you read the "article2" I linked at the beginning of this thread, or browse the list of references/cites at its end?

-Dave

914und993

That article you reference, by Fitch, is certainly alarming enough. Almost all of it is in reference to diesel engines, though, which have a much higher soot load than gasoline engines.

As for wear particles in the sub-10 micron range, isn't that what the metals concentration in an oil analysis is measuring indirectly? If those levels are 'normal', can you infer that your level of abrasive wear particles is low too? I don't have the answer to those questions.

The oil analyses I have had done, with one exception, did not include a particle analysis. That is an expensive addition because a guy has to sit at a microscope and take a look, and the lab said it was useful primarily in transmission fluids and not engine oil that has not shown a problem with high wear metal content.

You are fortunate in one respect: it is a heck of a lot easier to change the oil on the 964 than the 993. Maybe that is why I'm hoping so hard that shorter drain intervals don't gain much!

Chip

Christer

Well at the end of the day, regardless of how often you change the oil the engine will require attention - and at some expense most probably. We all know it and have to accept it. I would just do what you are happy with when it comes to changing the oil and at the same time put some money away for repairs/rebuild in the future, be they 2000miles or 200000 miles down the road.

Enjoy!