I have a car that has over 140k on it. Are there any valid reasons to use high milage oil instead of conventional oil? Or is it just a marketing gimmick?
Similarly what's the difference between standard oil and synthetic oil?
Cars are designed to operate with a minimum oil film thickness in all the contacts. This is a function of viscosity and oil feed rate. Heat up the oil too much and the viscosity reduces to the point where the oil film cannot support the contact and you have metal to meta wear (really bad). On the other hand if the viscosity is too high, then you cannot flow enough oil (i.e. cold starts) to support the contact also. Note that besides temperature, mechanical load (pressure) affects the viscosity. Strangely, idling a car a lot puts a lot of stresses on the mechanical contacts on the valve-train, and reduces the oil feed rate because the rpms are low. Remember the oil pump is crankcase driven.
So what does this have to do with what oil you use? Well the manufacturers and suppliers spend A LOT of time and money making sure the parts work well under with the oil specifications stated in the owners manual. Use the wrong oil and you might have trouble. As far as the synthetic vs. mineral oil goes, that is another topic to discuss elsewhere. Your best bet is whatever oil can maintain the required film thickness on the metal contacts the longest, and under all the varied environments we drive through. Synthetics are more stable and last longer, but they are not required usually unless you have a turbo car where proper lubrication is super-duper critical.
Regardless of the type of oil, the oil grade defines the viscosity, and thus how well it works with the engine. There are well defined SAE limits on this. A worn engine will have a tougher time supporting the right oil thickness because a lot of the asperities that retain oil will be worn out (polished away actually) so you might need a slightly thicker oil as time goes by, but then you have to be careful at cold temperatures. The high mileage oils may be tweaked for such conditions, or maybe it is just marketing. The important thing, again, is to retain good oil quality while the engine is operating.