Oil change interval, why is there a time limit?

Are we reading the same Onan manuals? I just looked at both the 4000 gas and the 7500 diesel and the book says change oil and filter at 150 hours same on gas but no filter but does say can be extended if synthetic.
Maybe we read at different levels.

LEN

T18skyguy wrote:
This is what I've read on the longevity of oil in the pan by date. The oil slowly combines with oxygen, and results in oxidation, which leads to sludge/coke or whatever other bad things. The filter media can also be attacked and ruined by the acids in oil while it sits in the filter. Then there is also the steady addition of water to the oil. This happens even when the engine is not running. The air in the engine is the same as outside, so when it hits the dew point, it condenses inside the engine, makes rust if it can, then settles into the pan. That water burns off if you run it, but not if it's parked for long periods. At night the engine inhales when the air gets cool(contraction), and when the day warms up it exhales(expansion), kinda like we do. I do the Mobil 1 annual oil once a year.

Right answer, finally! The moisture and acids that build up in the oil will slowly corrode the internal components of the engine. Its not that the oil breaks down, it doesn't; its the impurities that build up in oil that has some miles on it.

Chum lee wrote:
In many cases they put a time limit on oil life in addition to a mileage limit because if you don't use the vehicle much, it could be subject to excessive cold starts with very little accumulated mileage to warm the engine (oil) up fully. As previously mentioned, moisture and acids build up in the oil over time. When you have a lot of cold starts, unburnt fuel can also build up in the oil because of the temporary rich condition at cold start. That reduces the oil viscosity. Before electronic fuel injection became popular, that was a much bigger problem. Now, not so much but, many manufacturers still put a time limit on oil life. Manufacturers have no idea how their customers are going to use their product so in many cases IMO it just CYA.

When I was a kid, the oil in grandmas car was always overfull and looked like water. She only drove the 390 cubic inch 4 barrel carbureted monster to church on Sunday and to the local grocery store.

Chum lee

This is true. We had 2 very large, V16 diesel generators at our plant. We tested the oil for clarity and viscosity, every 3 months. The longer the oil was used, the less viscosity it had. Very seldom did we test for metals, as they were diesel engines, and metal in the oil was a fact of life, as the engine was made of steel, aluminum and various combinations of the above metals. Do what you want, as you own the engine. Starting the MH and not fully getting it warmed up, as in loading the engine/drive train by actually working it hard, like climbing hills. If you are not going to do this, do not start it.

supercub wrote:
. Why is there a time limit?

To sell more oil?

Oil sample. It is a non biased opinion of what condition your oil is in.

Time, “oil manufacturer” isnt obligated on my SUV warranty.

If the oil really went bad on its' own the oil manufacturer would confirm to change the oil at one year.

In many cases they put a time limit on oil life in addition to a mileage limit because if you don't use the vehicle much, it could be subject to excessive cold starts with very little accumulated mileage to warm the engine (oil) up fully. As previously mentioned, moisture and acids build up in the oil over time. When you have a lot of cold starts, unburnt fuel can also build up in the oil because of the temporary rich condition at cold start. That reduces the oil viscosity. Before electronic fuel injection became popular, that was a much bigger problem. Now, not so much but, many manufacturers still put a time limit on oil life. Manufacturers have no idea how their customers are going to use their product so in many cases IMO it just CYA.

When I was a kid, the oil in grandmas car was always overfull and looked like water. She only drove the 390 cubic inch 4 barrel carbureted monster to church on Sunday and to the local grocery store.

Chum lee

I would be happy to "recycle" your one year old, low mileage oil.:W

Cobra21 wrote:

T18skyguy wrote:
This is what I've read on the longevity of oil in the pan by date. The oil slowly combines with oxygen, and results in oxidation, which leads to sludge/coke or whatever other bad things. The filter media can also be attacked and ruined by the acids in oil while it sits in the filter. Then there is also the steady addition of water to the oil. This happens even when the engine is not running. The air in the engine is the same as outside, so when it hits the dew point, it condenses inside the engine, makes rust if it can, then settles into the pan. That water burns off if you run it, but not if it's parked for long periods. At night the engine inhales when the air gets cool(contraction), and when the day warms up it exhales(expansion), kinda like we do. I do the Mobil 1 annual oil once a year.

This answer makes a lot of sense to me! Thanks for the post.
Brian

And it's the right answer. :W

Everyone including myself have their opinions on the subject but what does Cummins say about Oil Analysis? Just an FYI.

From the Cummins Recreational Vehicle Quick Reference Guide on all of the engines I have seen:

"Oil Analysis

Oil analysis, as a method to extend drain intervals, is NOT recommended. Different methods of measuring soot, lack of correlation among testing labs, and differing driving patterns and idle time are the basis of the recommendation.

Oil analysis can be useful in detecting contaminants such
as dirt."