Motor oil longetivity and average engine RPM

[Zako2]

Is there a relationship between the motor oil life length and the RPM at which the engine usually runs?

This question is motivated by my observation that a lot of cars with older 4-speed automatic transmissions end up cruising on the highway at say about 2,500 RPM or above (this is at 60mph speeds). On the other hand, cars with more modern 6-speed transmissions keep their engine RPM below 2,000 revs at similar speeds.

Could this relationship explain why some car manufacturers have extended the oil change interval recommendations? E.g. Ford's recommendation for most of its cars before 2007 was 5000 miles or 3000 miles under severe duty. The 2008 scheduled maintenance manual extends this to 7500/5000 miles (and no longer than 200 hours under severe duty) without making an explicit requirement to the type of motor oil to use, other than it having a formal approval, such as WSS-M2C946-A (all? conventional oils have it).

 

[salv]

The only reason manufacturers are "extending" oil change intervals is because there is very stiff competition in the industry to have a product line with the lowest cost of ownership. Oil changes are a part of that cost of ownership. Fewer oil changes lead to lower cost of ownership. That is also why some manufacturers are covering oil changes for the first few years of ownership.
Potential buyers are now turned off to vehicles if the oil change interval for a given vehicle is not greater than 5k.
What the owner's manuals aren't telling customers, is that it is their responsibility to monitor oil level and condition over these extended intervals.

 

[The_Jeff]

My dad's 1980 fairmont was bragging in a brochure about 10,000 mile non severe conditions OCIs and how great that was vs 1970. I think they had unleaded gas to credit for that one.

Seems if you really want to preserve oil, don't shear it with tons of gears. Timing belt instead of chain for example.

The low cost of ownership thing is why everyone says do a timing belt/ major tune up at 105k... the first 100k are easy.

 

[heyu]

Originally Posted By: salv
The only reason manufacturers are "extending" oil change intervals is because there is very stiff competition in the industry to have a product line with the lowest cost of ownership. Oil changes are a part of that cost of ownership. Fewer oil changes lead to lower cost of ownership. That is also why some manufacturers are covering oil changes for the first few years of ownership.
Potential buyers are now turned off to vehicles if the oil change interval for a given vehicle is not greater than 5k.
What the owner's manuals aren't telling customers, is that it is their responsibility to monitor oil level and condition over these extended intervals.


I disagree on this one,,The reason why extended oil change interval recommendations is because FUEL INJECTION,,With the ol carburetor motors gas was always not getting smoothly burned and therefore eventually washed down past the rings and broke down the oil,,,

Fuel injection is much more accurate and does not overflow cylinders with gasoline and inputs only gasoline that is needed

 

[JAG]

For a given engine load, if RPMs are, say, 3000 instead of 2000, the oil's residence time in the piston ring area is typically shorter, and that has been found by researchers to reduce oil degradation rate.

 

[Trav]

You are both right.
VW IIRC started all this with 7500 OCI on the original beetle in the 70's and that was long before FI was common. For exactly the reasons salv put forth.

Today i think the reasons are two fold, basically what you both suggest.

 

[UNC151]

Perhaps, (and may be not applicable) but back in the days of carburetors, I had a 1980 Pontiac Trans Am that had 2.41 rear gearing and a TH350 transmission. At 65mph it "idled" at about 1600rpms--this was one of GM's efforts to increase MPG back then.

Although I am sure the fuel delivery system had a bigger impact on oil life versus engine RPM, the oil still had to be changed at 3K/3Mo.

 

[TFB1]

Originally Posted By: Zako2

This question is motivated by my observation that a lot of cars with older 4-speed automatic transmissions end up cruising on the highway at say about 2,500 RPM or above (this is at 60mph speeds).).

Not so, none of my 4.6L Fords with the 4-speed AOD or AODE have cruised at more in than 1700 at 65 MPH and that dates back to 1992... Same trans and rear ratio was used in many of the 80s 5.0 models incl, Crown Vic, Thunderbird Mustang, etc...

My '69 Fairlane with a 3.00 rear gear cruses around 2300 at 65mph and has no overdrive, period...

 

[ottotheclown]

Trouble with this idea used oil analysis show fuel dilution maybe not so much. I think the competion of long oci and your facts like others said, is the combined reason for 7500k. In addition tons of people do not ever think of oci. As a result the mfg. must tell them to do things. We on BITOG should figure this out ourselves. If your driving by corn

go for it on the other hand 4 miles is a long trip adjust to the shearing due to fuel dilution.

 

[modularv8]

There have been alot changes in engine design/hardware, and motor oil technology. Alot of today's engines have improved PCV systems, EGR elimination (VCT takes care of this function), low friction design (roller lifters), EFI, control systems (computer algorithms/sensors), etc. And much higher motor oil quality/OEM requirements all contribute to extending the OCI.

 

[JHZR2]

Originally Posted By: JAG
For a given engine load, if RPMs are, say, 3000 instead of 2000, the oil's residence time in the piston ring area is typically shorter, and that has been found by researchers to reduce oil degradation rate.


This is a good point, assuming that oxidative degradation is the main factor present. Given the oxidation resistance of Grp III/IV/V oils, and the relative lack of oxidative thickening that we see even in lower end oils these days due to add packs, Im not sure that aspect is the key driver...

 

[mechtech2]

Highway RPM are lowered to get more MPG.

OCIs are getting longer due to better engine design and manufacturing, tighter management/controls, and better oils.

 

[Lyman]

My Mustang turns 2,000 rpm at 65 mph. With the 6 qt sump I could probably do a lot longer OCIs on it. I stick to 5-6k OCIs only because Im so anal.