The modern four-stroke motocross bike engine is a complex miracle that pumps out more horsepower than engineers would have ever dreamed of even a decade ago. Believe it or not, there was a time in our sport when thumpers were considered too heavy and too slow to be competitive against their two-stroke brethren, but current valve-and-cam technology has all but rendered the once-dominant two-stroke obsolete. With more power, however, comes more internal heat, and engine oils are taxed more heavily than ever. Gone are the days when you could save a buck by strolling down to your local gas station to pick up a quart of automotive oil for your bike; the demands placed on engine oil by a motocross bike are far greater than an automobile. In addition to lubricating the top-end, valve train, cam, and clutch, engine oil in a four-stroke motocross bike must also deal with the transmission components, all the while withstanding the operating temperatures that can skyrocket during a hard moto. Oil temperatures in some race four-stroke bikes can reach up to 265 degrees, whereas gear oil in a two-stroke can hover below 200 degrees.

ADDITIVES

What makes motorcycle-specific engine oil different than passenger car oil? Additives—both the addition of an absence of. Zinc and phosphorus are important ingredients that render oil more resilient, but they have been reduced/eliminated in passenger-car oils in the late 1980s and in diesel oils in 2008 due to more stringent emissions regulations. Motorcycle-specific oils still contain these ingredients as they help the oil stand up to the greater demands of the wet clutch, transmission, and valve train. Furthermore, automobiles utilize a roller cam design that's much easier on oil than a motorcycle's flat tappet design that breaks down the viscosity of oil at a more accelerated rate. Passenger car oils also contain additives to help increase gas mileage, and these can adversely affect the motorcycle's wet clutch because these extra-slippery additives will cause slippage. Because of the constant aeration motorcycles subject their oil to due to lean angles in corners and the bouncing and bobbing in jumps and whoops, motorcycle-specific oils contain both antifoam and surface-active chemicals to maintain proper lubrication in all instances that a passenger car isn't subjected to. Lastly, premium motorcycle engine oils contain ester, a synthetic additive that is expensive but provides vastly superior lubrication and heat dissipation. Ester is widely regarded as the most effective racing oil additive, and many top-of-the-line motorcycle oils will utilize its benefits

GOOD GOLLY MISS MOLY

Molybdenum disulphide is a super-slick additive that's found in some very specific motorcycle oils made for motorcycles equipped with separate engine oil and transmission oil tanks. It should never be used in Husqvarna, Kawasaki, KTM, Suzuki, or Yamaha bikes, because it will cause clutch slippage. Only the current Honda CRF250R (as well as some street motorcycles) currently have a separate tank for engine oil, and the Honda CRF250R can benefit from engine oil with a moly additive because it isn't circulated into the wet clutch or transmission. Transmission-specific oil should be used in the gearbox of the Honda CRF250R, as transmission oils are formulated specifically to protect transmission gears, reduce noise, and aid in smooth shifting. Honda Pro Oils and Lucas Oils are the largest proponents of engine oils infused with moly.

A WEIGHTY PROPOSITION

We’ve all seen the numbers 10W-40 or 15W-50, but what do they mean? The first number in the code designates the oil's ability to pump at extremely low temperatures. For instance, 5W oil will pump through a cold engine much easier than 20W will. The more important number in the code is the second number because it designates the oil's viscosity at 100 degrees Celsius, which is close to the operating temperature of a motorcycle engine. For motocross usage, all of the experts we consulted agreed that 10W-40 is the ideal weight for most applications. For high-performance racing applications, 5W-30 may also be used.

SYNTHETIC OR PETROLEUM

Professional race teams rely on synthetic engine oils, but should you? In our survey of oil technicians and mechanics, the consensus was that both work great for motocross applications and are equally ideal when changed on a routine basis. Petroleum oils are less expensive, but lazy riders who run their oil longer, or riders who do long-distance rides or races may benefit from the greater resilience and superior lubricating properties of synthetic. On the other side of the coin, riders who abuse their clutch or are riding with well-worn plates may experience clutch slippage sooner with synthetic oil due to its slicker consistency.

MAINTENANCE

How often should you change your engine oil? That is determined largely on how you ride and what conditions you ride in. Heavy clutch abusers and those who ride in mud or sand should change their oil more often because it becomes filled with solid particulates worn off of the clutch plates. Most professional race teams change the engine oil every moto, but for most riders, every two to three ride days is acceptable. Changing your engine oil more often than that, however, is not overkill—it's investing in the long-term durability of your expensive machine. But how often should you change your oil filter? Every other ride was the consensus of the experts we consulted. A good gauge is to get a small amount of oil on your finger as you're draining your engine and check it for any amount of clarity. If it's so dark that you cannot see through it, that means that it's loaded with solids and the oil filter should be changed, as the solids are abrasive.

OIL IS OIL

One of the most common misconceptions about oils is that they're all the same, within their specific category, but truth be told there are more than three categories: There are five different categories of base oils. Group I and II are petroleum, with Group II being a more pure, higher quality grade. Group II petroleum oil will blend better with antifoam and surface-active chemical additives, and this is the most commonly found petroleum oil in motorcycle applications. Groups III, IV, and V are all synthetic, with Group III being a highly processed petroleum-derived oil that's most common in a passenger car application. Group III oils suffer from low-temperature viscosity and poor pumpability in low temperatures. Group IV and V synthetic-based oils are most commonly utilized in motorcycle applications. So what's a semisynthetic? A blend of Groups I or II, with III, IV, or V. In a survey of what most professional motocross racing teams use in their race bikes, the answer was most commonly synthetic for their superior lubrication qualities.

HOW MUCH OIL DOES MY FOUR-STROKE TAKE?

Below, you will find the recommended oil capacities for all current four-stroke motocross bikes. As a rule, you should replace your oil filter every other oil change, but serious racers do so with every oil change. When you remove a dirty oil filter, more oil will pour from the cases. That said, more oil is replaced when performing a complete service including a new oil filter. Furthermore, many mechanics we spoke to suggested priming the oil system by filling the new oil filter with a few ccs of new oil before installing it.