What causes oil consumption to progress, further into the OCI?

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Aug 14, 2015
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I have been tracking oil levels/consumption in the Sienna & Highlander across several OCIs. I have noticed that it does get worse as the OCI progresses. For both cars, the first 1000-2000 miles, there is very little. But towards the end of the OCI, the oil level does drop faster. I have seen this phenomenon more than once, for both those vehicles.

The last several oil changes, I have used 20 grade oil in them. Both cars were originally 5w30, but were back-specced to 20 grade. With the Highlander, where I have seen consumption the greatest (just over 1.5 quarts in 5-6K miles), I'll move to a thicker oil next time I do change it. I have a ton of 20 grade oil in my stash, so I will probably want to use it up before moving the Sienna to a thicker oil, unless I see a big increase in consumption with that engine as well.

Now for my question: Does this non-linear consumption point to any one thing as being the cause of oil consumption (as in seals or piston rings)?
 
Last 2 UOAs I had done, fuel was 2.3 & 2.6%. However, viscosity (100C cst) was 8.5 & 8.3, so it couldn't have sheared too much, or lost too much viscosity throughout the OCI. OCIs were 5-6K miles.
I would also think the oil sheering and/or fuel dilution would make it even thinner and able to pass the rings faster.
 
I have noticed the same with all oils, regardless of brand. I think it has something to do with contaminant loading.

Consumption is always the lowest for the first 2-3K, then sharply increases.
 
Shears/gets thinner, easier to leak past rings & valve seals, to be burned in combustion. Would think NOACK goes up too, more evaporates & gets pulled into the PCV valve.
Noack will go down over a run, because the lighter fractions will have boiled off the longer a given quantity of oil is in use. But in turn, the viscosity will increase and the winter rating will also suffer.
 
When you say "contaminant loading," are you talking about particles coming loose from the filter?

I have noticed the same with all oils, regardless of brand. I think it has something to do with contaminant loading.

Consumption is always the lowest for the first 2-3K, then sharply increases.
 
Saw this a lot at work. Mid 00s Chevy vans. They would make 3000 miles easily without any noticeable consumption. Over 4000 consumption would skyrocket. I attribute it to the vehicle telling us it wanted an oil change.
 
I have seen that theory. Problem I have with changing it when oil consumption starts to increase, is that to the extent that my measurements are accurate, I am seeing the increase in consumption a bit too early for an oil change in my opinion. It is starting to greatly increase NLT 3000 miles.

Saw this a lot at work. Mid 00s Chevy vans. They would make 3000 miles easily without any noticeable consumption. Over 4000 consumption would skyrocket. I attribute it to the vehicle telling us it wanted an oil change.
 
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Hard to measure accurately with recent vehicles that have abandoned the dip stick for electronic measurement. I never add the full amount of recommended oil in an oil change. Only replace to the nearest quart. So, for instance, at a required full oil change of 6.3 quarts I only add 6 quarts. Even if I wait half an hour for a complete drain. Guage always shows "Full" after 5 to 7,000 miles.

Frustrating to have to wait for a warm engine to check oil level. Contemplating starting a "Bring back the dip stick!" campaign.
 
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