Do you know how often to change engine oil? Should you use synthetic oils? Is 10W-30 oil always a better choice than 5W-30?

“We frequently get questions from farmers about oils and protecting engines,” says Fred Whitford, head of Purdue University’s Pesticide Programs. “Proper maintenance of oil and oil filters is key to helping tractors, sprayers and combines last as long as they should.”

Whitford sought information from experts and compiled PPP-124, “Engine Oils and Their Filters — Understanding Oil Lubrication and Contamination Control.” It’s available at edustore.purdue.edu.

Here are five things you should know about engine oil included in the new publication:

1. Start and end with the owner’s manual. The key to selecting the correct lubricants for your equipment begins and ends with the owner’s manual, Whitford says. That’s true for a $500,000 combine and a $300 push lawn mower. If you don’t know what’s in the owner’s manual, you could void the warranty by not following recommendations.

2. When to change oil and filters varies depending on several factors. If you wait too long to change oil, you may be getting less engine protection from it than you think. Oil doesn’t degrade in a linear fashion, Whitford says. Instead, there’s a sharp drop-off in engine performance once oil is past its useful life.

Short-trip drivers should probably change oil more frequently. Older vehicles also require more frequent oil changes. Follow the manufacturer’s recommendations, and change the oil and filter at timely intervals to avoid permanent engine damage.

3. Consider testing engine oil. Some oil suppliers can test oil. If they can’t, they should be able to refer you to someone who can. Tests on used oil reveal if it contains antifreeze, soot, fuel, chrome or other contaminants that can damage the engine. Test results can guide you as to when oil should be changed earlier or later. Chrome in the oil could indicate abnormal bearing wear.

4. Know how synthetic oil compares to conventional oil. There’s a consensus that synthetic oils may help under extreme conditions, such as if you don’t change oil and filters regularly or when the engine coolant is weak, Whitford says. Under normal maintenance, driving and operating conditions, there is likely little difference between conventional and synthetic motor oils. A Consumers Union study of New York City taxi cabs comparing cabs running on each oil found no difference in engine wear after 100,000 miles if the taxi cabs received routine oil changes.

5. Understand what oil viscosity numbers mean. What’s wrong with this statement? “That 5W-30 oil is too thin; instead, let’s use the 10W-30 oil.” Whitford says someone who would make that statement doesn’t understand how oil viscosity numbering systems work.

The front number — 5 or 10 — indicates how the oil functions when the engine is first started. Lower numbers mean lower viscosity, or thinner oil, at colder temperatures. The back number indicates the viscosity of the oil at operating temperatures within the engine.

What’s wrong with the statement, Whitford notes, is that the back number, 30, is the same in both oils. That means both oils would have the same viscosity at higher temperatures.

In fact, the W in the designation doesn’t stand for “weight,” as some people think. It stands for “wintertime.”