High milage motor oil has extra additives which help prevent oil leaks.
Synthetic oil is synthetic oil. All brands are made from the same basic formula that has been around since World War 2. There is no real difference in any of the major brands. Buy the cheapest major brand. BTW, you will see no appreciable difference in mileage using synthetic versus conventional oil.
I recommend a high-mileage synthetic. 10w-30
yes, but it has to be synthetic oil
No, you can even mix synthetic with conventional oil. It will do no harm at all.
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Both
Several companies make a motor oil specifically for high mileage cars. Also a good synthetic oil might be just as good.
I work for a lube distributor. It is fine to go from synthetic to synthetic blend. Here's the difference: No manufacturer will actually release how much synthetic is in their synthetic blend formula. It may actually be closer to a conventional oil than a synthetic oil. The benefit of synthetic is that it offers better protection for your vehicle/ machinery. The secret is that it only makes a true difference if you're one of those people who doesn't change your oil regularly. If you choose to go synthetic blend, make sure you keep up with how often you change your oil!
Synthetic oil can be used in older motors, but if you have any small leaks, they will get significantly worse. I'd recommend a High Mileage blend like Valvoline Max Life or something similar.
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synthetic motor oil doesn't break down nearly as fast as conventional oil it also can have greater lubrication and anti rust protection.