Taste the oil if you must. The state/condition of the oil dictates WHEN you should change it. Even Toyota (imagine that) recognizes this. They say:
What are the oil change intervals using synthetic oil?
If the vehicle operation meets the standard criteria for "Special Operating Conditions" such as: driving off-road, on dirt roads, towing a trailer, making repeated short trips under 32˚ F, or extensive idling; the engine oil must be replaced at 5,000 mile intervals, regardless of what type of oil is used.
So it would appear that different driving conditions warrant DIFFERENT change schedules. Even the Toyota University advises that the 5th Gen...
"The intervals may change based on driving habits and conditions." This can be found here:
http://www.settraining.com/content/course-codes/12_07_HS_Toyota_Engines_and_Synthetic_Oil1.pdf
If you are NOT physically looking at your oil, then the 10K and 5K intervals are a good benchmark. But I'm sure we all check our engine oil at more regular intervals. During those regular visual inspections, you can see the condition of it. Are you going to tell me that if you looked at your oil and found it to be sludgy, you'd just put the cap back on and say "Toyota says that the 5th Gen T4R engine is good for a 10K oil change interval, I'll just wait?"
I don't think so, and that was precisely what I was advising in my post. Lets take a quick gander at what others "see" in oil:
HowStuffWorks "Checking Oil Clarity"
But as the oil gets older, the number of deposits in the oil continues to build and the filter is unable to extract all of the material. When this happens, the color of the oil changes and it indicates the oil should be changed. To check the clarity of the oil, let the car run for a few minutes and then shut it off. Find your oil dipstick, pull it out and check the level of oil and its color. If the oil is a dark-brown or black color, like Coca-Cola, then your oil needs to be changed. However, if it appears to be lighter, like the color of a cup of tea, then you still have some time before it needs to be changed.
How to Check Your Vehicle's Oil Level - For Dummies
Oil turns black pretty quickly, but that doesn’t affect the quality. Rub a little between your thumb and index finger, and if it leaves a dirty smudge, it probably needs to be changed.
How to Tell if Your Car Needs an Oil Change | eHow
Examine the oil on the dipstick and on the rag. There are three things you should check. First, look at the color of the oil. Fresh, clean oil is a light amber color and is transparent. Dirty oil is black or dark brown. If the oil looks dirty or is no longer transparent, it's time for an oil change. Second, look for any flecks of metal or debris in the oil. They might be barely visible, especially if the oil is dirty, but any debris in the oil indicates the need for an oil change and possibly some other engine problems.
Smell the oil for any burnt odor. Some mechanics will touch a finger to the dipstick and taste it for any sign of burning, but not everyone will want to taste their oil. Try smelling it instead. If you detect a burnt scent, it's time for an oil change.
oil change - How can you tell the oil needs to be changed? - Motor Vehicle Maintenance & Repair Stack Exchange
Grab the dip stick and run it through your fingers to get them oily. Spread the oil on your fingers and observe:
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Transparent, honey-like colour, no visible sediments, nice greasy feeling? You are good to go.
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Transparent, dark-brown to black, no sediments? Consider changing the oil soon.
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Murky, sediment-y, dark black? Change ASAP.
Engines running on flexifuel/ethanol (aka E85) can preserve the oil longer due to cleaner combustion, and it would normally have lighter colour than a comparable gasoline engine at the same mileage after the last oil change.
Checking Your Oil
If your oil is very black and dirty it could probably do with a change, although it can be quite normal for diesel engines to turn their oil black quite quickly. Healthy oil in a regularly serviced petrol engine is usually a light golden colour.
Dipstick Oil Analysis
Lesson No. 4 - Signs of an Aged Oil
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Oil is not like a fine wine that gets better over time. Instead, it ages at a rate that is influenced by driving conditions, fuel quality, engine age, motor oil quality and climate. If not changed in time, your oil will wither and fail to protect your engine.
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So, let’s take a close look at the oil on the dipstick. The oil should look smooth and glossy and somewhat transparent. If it has sludgy deposits or grainy particles of dirt, it’s time for an oil change. The same is true if the oil looks too thick, is too dark (opaque), and/or has a putrid rotten-cheese smell.
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If you still don’t know whether you need an oil change, consider doing a blotter spot test.
Oxidized and contaminated oil will lose interfacial tension. A simple test for interfacial tension is to place a drop of used oil from the dipstick on the surface of water. If the oil drop spreads out over the water’s surface (instead of beading up like a new oil) it may be time for an oil change.
You advised me that a lot has changed in the last 30 years with regard to oil and engines. I have an open mind, please tell me what has "changed" in the last 30 years?
