Stand in the oil aisle and the labels look like a code: 5W-30, 0W-20, full synthetic, high mileage. Pick the wrong one and you can hurt fuel economy or cold-start protection. The good news is the code is simple once you break it down.
Reading the Viscosity Grade
Take 5W-30. The first number with the W is the cold rating; W stands for winter. A lower number flows better when the engine is cold, which matters most at startup when wear is highest. The second number is the viscosity at full operating temperature. A 30 stays thicker when hot than a 20 does. So 0W-20 flows easily on a freezing morning and runs thinner when warm, while 5W-30 is slightly heavier on both counts.
Use What the Manufacturer Specifies
Your owner's manual lists the exact grade the engine was designed for, and modern engines with tight tolerances and variable valve timing are picky. Putting 5W-30 in an engine that calls for 0W-20 can cost you a couple of miles per gallon and confuse the oil-pressure system. Match the manual, including any specification code like a dexos or manufacturer approval printed on the bottle.
Synthetic vs Conventional
- Full synthetic: more stable across temperatures, resists breakdown, and supports longer intervals; required on many newer engines.
- Conventional: cheaper and fine for older designs with short intervals.
- High mileage: adds seal conditioners that can slow minor leaks on engines past 75,000 miles.
Don't Forget the Filter and Interval
Fresh oil in a clogged filter does only half the job, so change the filter every time. Follow the manual's interval, typically 5,000 to 10,000 miles on synthetic, and shorten it if you do lots of short trips or towing. The right grade, changed on schedule, is the cheapest insurance your engine will ever get.