Synthetic oil used to be something only performance ... The alphanumeric codes on a quart of oil—5W-30, 0W-20, etc.—denote its “weight,” or viscosity. Thicker, higher-viscosity liquids ...

Stick with the manufacturer's recommended motor-oil viscosity classification. For instance, if your car uses 5W-30 synthetic oil, upgrade to high-mileage oil with a similar 5W-30 rating.

the only motor oil you have available is 5W-30, and you must drive it, it's better to add the 5W-30 than it is to drive your car with extremely low engine oil. However, I would advise against ...

5W-30 and 0W-40; hold potential for higher temperature performance (typically applies only to all-synthetic heavy-duty engine oil formulations). “There is no doubt synthetic oils make a better ...