Its difficult for everyone in determining the oil's health by mere inspection from a dipstick and check for the viscosity etc.Are there any other symptoms which a vehicle craving for new oil would show like reduced performance,pickup etc.
There's a very simple way to tell ... In most modern vehicles there's an oil life monitor. Follow it and you cannot go wrong (as long as it's reset at the last oil change). Manufacturers put some time/effort/money into getting them right. They measure a lot of the properties you talked about as well as other factors you didn't. It knows how many miles you've put on the vehicle since the last time it was reset. It knows how high you've revved the engine and for how long. It knows how much strain you've put on the engine during the oil change interval. It computes all of these things and spits out a time when you need to change it. The best thing about it is, following it also covers the manufacturer's warranty for the vehicle. As long as you check it regularly to ensure the oil is up to level, you're golden. Change the oil the next time it tells you.
Over the course of thousands of miles, as the oil loses viscosity, you'll notice a reduction in oil pressure, especially at idle when the pump is going slow. You're not going to notice acceleration differences unless the bearings surfaces are touching. If they were, you would know it.
If oil pressure is over 10 psi at idle (and higher through the rev range) you have nothing to worry about. Keep in mind that the average car does not have an accurate oil pressure gauge. An idiot light is the most popular solution.
If it's just an ordinary modern street car, changing the oil every 4-5000 miles is plenty. Just don't let the oil level get too low for the pump to work.
There are several possibilities on how an engine oil will get degraded up to the point where it will be "unfit"
IMHO (in a very rough sketch) the most important factors are:
- Breakdown of viscosity: Not enough abrasion protection
- Depletion of the additives: Lack of abrasion protection and cleaning capacity
- Dirt saturation: Lack of cleaning capacity
- Base depletion: There is no base left that can neutralize acid combustion byproducts, the acid attacks the engine
- In case of diesel engines (especially particle filter equipped engines) there exists the danger of excessive diesel contamination: The oil gets too thin and lacks abrasion protection.
My personal oil-change strategy is quite simple (ymmv):
- I follow the manufacturers guidelines (my car has an oil life indicator). On 1/4 of remaining oil life I replace it.
- From time to time I inspect the cylinder head surface through the filler opening: Should I find beginnings of dirt buildup on the metal surface it means that the oil is depleted.
On average I change my oil every 12-15kkm. Oil color is of no concern for me since I drive a Diesel, the oil is always pitch black. While I cannot determine the engine wear a perfectly clean engine head convinced me in my strategy.
One last thing: Should your dipstick show a oil induced lacquer-like hard coat the oil is definitely degraded