Are SAE 20W-50 or SAE 10W-40 grade motor oils best for engines?

Once thought to be more protective, heavier engine oils like 20W-50 and 10W-40 are now shown to be less effective than lighter oils. With heavier oil grades, the oil cannot flow fast enough to give good lubrication in cold weather – especially on start up. Most auto manufactures now recommend using lighter grade oil in newer cars.

I've heard that I can't use the same oil in all my diesel equipment.

Today's modern API CJ-4 oils are acceptable for use in almost all North American diesel engines and often carry multiple OEM approvals. Additionally, these oils often meet API gasoline engine requirements and are acceptable for use in many heavy duty gasoline engines.

For diesel engine oils, TBN is good; the higher the better.

TBN is good, but it is only one of several oil properties that are important for engine life. Other important properties include the inhibition of acids and oxidation, maintaining viscometric control at high soot levels, low wear and maintenance of oil film strength. All of these properties can affect oil life. Additionally, high TBN oils often contain high levels of ash, which may impact emissions control devices.

I've heard bulk oil is of a lower quality than bottled oil.

If you are going to a reputable mechanic, the bulk oil should be the same quality as the bottled oil. If you take your vehicle to a mechanic for an oil changed, ask what type of oil they are using. Make sure the motor oil used is equal to the quality level listed in your owner’s manual.

Motor oils made with certain base stocks are better than others.

There are four types of base stocks used in motor oil, and there are differences between each. Group I base stocks use the oldest refining techniques. Group I oils are generally less capable of formulating into high quality motor oils than Group II, III and IV oils. Groups II and III are commonly used as base stocks for motor oil with good performance in key properties such as volatility, oxidative stability and flash / fire points. Group III is the highest level of base stock which is not chemically engineered. Group IV base stocks are chemically engineered, known commonly as synthetics. When combined with additives, this base stock is highly stable and has highly uniform molecular chains. To ensure quality when picking motor oil, look for the seal of a nationally recognized petroleum council.

Motor oils can cause sludge and deposits.

This myth does have some truth to it. Poor quality oil can contribute to the buildup of sludge because lower quality oil does not properly clean the surface areas of your engine. Over time, dirt, soot, oxidized oil, leaked coolant and other elements can build up on your engine, resulting in a decrease in performance. On the other hand, high quality motor oil should help to break down these particles, allowing them to be drained with the oil during regularly scheduled oil and filter changes.