A tough question. In addition, one person trusts the car only to highly qualified specialists, the other - to anybody, the third one does everything with his own hands. Everyone has their own philosophy and economics.
There is an injection VAZ 2109 on my list. Its owner is a very demanding motorist; he will not go on a ruin. When the mileage reached 200 thousand km (!), The owner pounced on me: “The engine is in order, but please upgrade it - I’m paying for everything!” When disassembling the engine, we were more amazed, not finding any objective prerequisites for repair. Even the mesh of flat-top honing is preserved, the dimensions of the crankshaft necks remained in tolerances. There was no hint of black dirt under the timing cover. The reason for the miracle turned out to be trivial: the owner poured into the engine only expensive synthetic oil … - and thereby, it seems, deliberately “increased” the operation of the machine. Eccentric?.. Do not rush to judge! Today, mileage is already about 300 thousand, and at that time we only set the pistons to 0.01 mm “fuller”, rings and inserts in the bargain. The "stubborn" owner continues to buy synthetics - and nightmares involving collapsed equipment do not threaten him.
But many argue differently: let the "horse" consume what they give - even straw instead of oats. We have repeatedly had to restore the engines, "killed" with a range of 60-80 thousand km, and a full repair program can cost the owner of thousands of fifteen or even more. Well, consider how much such an operation will result in a mileage of 200, 300 thousand - at least in cheap oil! And who will take into account moral losses - battered nerves, time and effort spent on repairs, etc.?
Such cases are indisputable, and we immediately issue the verdict “repair!”. The first duty is to check the compression in the cylinders. The main sign of trouble is a large (over 4–5 kgf / cm2) spread. In this case, fill the cylinder with low compression with 15–20 ml of oil and repeat the measurement. The indicator jumped one and a half to two times? We will sympathize with the owner: the cylinder-piston group is out of order, a serious repair is ahead. If the oil injection is weak, there is a “hole” in the valves. Perhaps the repair will be limited to the head of the block, and not necessarily entirely: it is often possible to reduce it to individual valves and seats.
The increased emission of oil from the crankcase ventilation into the intake tract indicates the wear of cylinders, pistons, rings. For example, in the air filter housing on carburetor machines. But it’s more serious at injection ones: there is a mass air flow sensor that is very sensitive to pollution.
Blue exhaust smoke? It is necessary to carefully study its nature. Smokes at idle, and almost disappears when gas is left, which means that we are preparing to replace valve stem seals: cylinders or pistons, most likely, nothing to do with it. The oil on the thread of the candles is often also a sign of trouble with caps. Changing them is not so difficult, but buying good caps is a great success.