Manufacturers rarely give you just one answer when it comes to motor oil. You will often see two or more viscosity grades listed in your manual, such as 5W-20 or 5W-30. They do this because engines operate under wildly different conditions. Temperature is a huge factor. Knowing how viscosity actually works helps you pick the right fluid instead of guessing.
Viscosity is simply a fluid’s resistance to flow. In the oil world, we use the XW-XX notation. It looks like a code, but it is standardized. The number before the “W” rates flow at 0 degrees Fahrenheit (-17.8 degrees Celsius). The “W” stands for Winter. It does not stand for weight. Many people think it does.
A lower number means less thickening in the cold. 5W-30 thickens less than 10W-30. It thickens more than 0W-30. If you live in a cold climate where oil turns to sludge, 0W or 5W viscosities are your best bet. Drive a car in Death Valley? You need a higher number. The oil must not thin out until it loses its ability to lubricate.
The second number measures viscosity at 212 degrees Fahrenheit (100 degrees Celsius). This number tells you how the oil resists thinning at high operating temperatures. 10W-30 thins out faster at high heat than 10W-40. Stick to the range in your owner’s manual. Work within those parameters.
Once you know the viscosity, shop for the type of oil. Most commuters follow the 3-month and 3,000-mile rule. Frequent changes mean there is less need for specialty oils compared to conventional. Some brands, like Mercedes-Benz and BMW, only recommend synthetic oil. Use the list below and your manual as a guide. Do not switch between types randomly. If your car started with conventional, stay with it. If it uses synthetic, be wary of switching back to conventional.
Conventional vs. Synthetic Oil Options
Conventional Oil
This is the bulk oil used at dealerships. It is the cheapest at the auto store. Most meet API and SAE standards. They offer little in the way of additive packages. It is good for owners who change oil religiously. It also suits low-mileage, well-broken-in engines.
Premium Conventional Oil
This is the standard new-car oil. Leading brands have one for SL service. These are available in common viscosities. Car manufacturers usually specify 5W-20 or 5W-30. Some require 10W-30. These three ratings cover almost every light-duty vehicle. This is changing as engines become more precise. They are fussier about specific oil types.
Full-synthetic Oil
These oils are made for high-tech engines. If they pass stringent tests, the labeling reflects it. They have superior performance. They last longer. They protect against engine deposits better. They flow better at low temperatures. They maintain peak lubrication at high temperatures. Synthetics are about three times as expensive as conventional oil. They are not always necessary. Use your manual as a guide. If it does not call for synthetic, using it is just an added expense. It may not improve performance or engine life.
Synthetic-blend Oil
This is premium conventional oil with a dose of synthetic. It is formulated for better protection during heavy engine loads. Higher engine temperatures are common here. These oils are popular with pickup and SUV drivers. They offer better protection. They usually cost only a fraction more than premium conventional oils.
High-mileage Oil
More than 60 percent of vehicles on the road have more than 75,000 miles on the odometer. Refiners developed high-mileage oils to target this market. Seal conditioners are added to the oil. The oil can be synthetic or conventional. The conditioners expand and increase the flexibility of internal engine seals. They are very precise. They benefit some engines. They do not affect others.
Some companies also add viscosity modifiers to thicken the oil. They add anti-wear additives too. We will discuss those on the next page.
