Dirty Oil Question

[chinee]

A question was asked about dark colored oil on a G35 board and this was one of the responses:

quote:

---

Q45tech (Registered)
06/12/04 10:10 AM

Depends on the sizes of the dirt particles.
Oil analysis doesn't measure particles as to size.
Experiment just change the filter if the oil cleans up some in 50 miles you know!

The G35 filters are tiny and have a limited hold capacity, this is the limiting factor not the oil stability.
Why Nissan warns for longest engine life change every 90 days. What they are saying is if you want the engine to perform well at 200,000 miles follow this interval.

Those that are leasing or care nothing for future owners can get by with 6 month [7500] changes.

"A cold start can also be a dirty start, because of the oil pressure-release valve in your oil filter. If the oil pressure is too high (say, if the oil filter is clogged up, or you have thick oil), this valve opens, and unfiltered oil enters the oil circuit of your engine. The crankshaft bearings usually get their oil directly from the filter, and so they usually suffer first. "
http://www.baldwinfilter.com/engineer/pdf/89-5r.pdf
http://www.websorcerer.com/GeekOut/
http://www.baldwinfilter.com/engineer/95_11.html

A 25 micron filter will only filter 0.001" and the G35 bearing clearances mean you need to filter to 8-10 microns or better.
Compared to some domestic which may have 50-60 micron bearing clearances.
http://www.shoclub.com/lubrication-oil/lubrication-oilpart6.htm

Every time you exceed 3,000 rpm the oil pressure rises to the bypass point and unfiltered oil get circulated, as the filter gets dirty the rpm bypass point gets lower and lower.

People that do lots of high rpm cruising, run into this problem in coolish temperatures.

---

I've never read this type of feedback over here and wanted to know what bob's regulars think. Is there validity in his explanation?

His response mentions crankshaft bearings being the first to suffer from dirty oil.. and I've had relatively high lead wear in my G35, so I was thinking that this statement would apply in my case...

Should I believe it?

 

[cryptokid]

not true. the oil that is in the sump was JUST filtered. remember this oil gets filtered over and over and over and over and hardly nothing actually gets into the oil in one pass. the oil in the sump is probably 99.99% as clean as the oil that just went through the filter.

 

[chinee]

What about if the filter is clogged and the oil flows through the bypass valve... this oil would be unfiltered, right? If so, would the crankshaft bearings be the first place it flows to? (I know almost zip about engine internals)

 

[ekpolk]

quote:

---

Originally posted by cryptokid:
not true. the oil that is in the sump was JUST filtered. remember this oil gets filtered over and over and over and over and hardly nothing actually gets into the oil in one pass. the oil in the sump is probably 99.99% as clean as the oil that just went through the filter.

---

You're right about what that guy said, but what you said is partially incorrect as well. The sump contains the oil that has drained down from whatever lubrication function it performed on its last pass (assuming we're talking a wet sump system). It also will contain a certain amount of just-filtered oil that has been let past the pressure control mechanism. Overall, it will be a mix of filtered and unfiltered oil.

 

[jsharp]

Is he trying to say Nissan bearing clearances are 10 microns? That's .0003937" for the metric impaired. I think I'd like to see that in a shop manual, not that I'd doubt an expert on the internet...

[ June 12, 2004, 11:18 PM: Message edited by: jsharp ]

 

[Leo]

Thats why I think sump magnets etc are good cos they help trap any particles that the oil filter doesnt!

 

[TC]

I wouldn't consider oil in the sump to be clean since (among other reasons) contaminants including soot can blow past the piston rings directly into the crankcase. There's good reason the filter is located directly following the pump and immediately prior to engine circulation. Oil changes every 90 days seems quite excessive to me except for truly Severe Service. The size of the filter can does not indicate actual media area -- I believe Subaru filters are small on the outside but often pack substantial total area. Don't know about G35 engines, but I'd be surprised if their clearances were dramatically less than equivalent competitors' engines. Non-clogged filters running in bypass is common and not a problem, such as at high-RPMs, cold engines, throttle jabs, etc. Per Grease's study, the PureOne offered the finest filtration at down to 11 microns, so "8-10 microns or better (required)" probably ain't gonna happen. Even the Wix and Baldwins (great quality filters) basically didn't filter below 14 microns with any consistentcy.