Publication #6SA314-8901

Function of Lubricating Oils

All diesel engines require heavy-duty lubricating oils. Basic requirements of such oils are:

    • Lubricating quality
    • High heat resistance
    • Control of contaminants


Lubricating Quality

The reduction of friction and wear by maintaining an oil film between moving parts is the primary requisite of a lubricant. Film thickness and its ability to prevent metal-to-metal contact of moving parts are related to oil viscosity. The optimums for Detroit Diesel® two-cycle engines are SAE 40 grade oils.

High Heat Resistance

Temperature is the most important factor in determining the rate at which deterioration or oxidation of the lubricating oil will occur. The oil should have adequate thermal stability at elevated temperatures, thereby precluding the formation of harmful carbonaceous and/or ash deposits.

Control of Contaminants

The piston and compression rings must ride on a film of oil to minimize wear and prevent cylinder seizure. At normal rates of consumption, oil reaches a temperature zone at the upper part of the piston where rapid oxidation and carbonization can occur. In addition, as oil circulates through the engine, it is continuously contaminated by soot, acids, and water originating from combustion. Until they are exhausted, detergent and dispersant additives aid in keeping sludge and varnish from depositing on engine parts. But such additives in excessive quantities can result in detrimental ash deposits. If abnormal ash deposits form on the exhaust valve seats, early engine failure may result. The oil that is carried up the cylinder liner wall is normally consumed during engine operation. The oil and additives leave carbonaceous and/or ash deposits when subjected to the elevated temperatures of the combustion chamber. The amount of deposits is influenced by the composition, additive content, engine temperature, and oil consumption rate.

Oil Quality

Oil quality is the responsibility of the oil supplier. (The term “oil supplier” is applicable to refiners, blenders, and re branders of petroleum products). Oil quality can also be affected by handling cleanliness, contamination, dirt, water, etc.

Lubricant Recommendation for Detroit Diesel® Two-Cycle Engines

    • SAE Viscosity Grade 40
    • API Classification: CD-II, (Note: CD-II is an obsolete designation replaced by the current API Rating CF-2)
    • Military Spec: MIL-L-2104D
    • Sulfated Ash: Less than 1.0%


This is the only engine oil recommended for Detroit Diesel® two-cycle engines. Lubricants meeting these criteria have provided maximum engine life when used in conjunction with recommended oil drain and filter maintenance schedules. (For additional oil selection criteria, see publication 7SE270.) The API category CD-II (CF-2)(Note: CD-II is an obsolete designation replaced by the current API Rating CF-2) is relatively new and may not be fully in use at the time of this publication. (NOTE: This publication was published in 1989) API category CD may be used provided the recommended military specification is satisfied. Oils with API designation “CE” are not recommended in DDC two-cycle engines unless accompanied by CD-II (CF-2) (Note: CD-II is an obsolete designation replaced by the current API Rating CF-2)

Single Grade-High Ash Content Lube Oils

High ash oils (greater than 1,000 mass % ASTM D-874) tend to form excessive deposits on engine parts, resulting in impairment of their function (exhaust valve closure, for example). These are NOT recommended, except where the continuous use of high sulfur content diesel fuels (greater than 0.5 mass %) is unavoidable and where a high TBN, LOW ASH oil is not available.

Multi-Viscosity Oils

Less than optimum engine life must be expected when using 15w-40 oils. Multi-vis oils tend to break down due to mechanical and thermal stress which results in a loss of viscosity and premature engine wear. Multi-vis oils do help cranking when temperatures are below 32 degrees F (0 degrees C). However, once the engine starts, the major advantage of the multi-vis oils is exhausted. When the engine reaches operating temperature, the internal engine temperatures and pressures vary little from an engine operating in much warmer climates. This is when the 2 cycle engine benefits from the superior protection of a 40 wt.

Starting aids such as automatic ether systems and /or on-board auxiliary heaters provide more reliable starting.

Engine Parts Can Tell The Difference in Oils

The following oil performance parameters should be compared:

    • Ring Sticking tendencies and/or ring conditions
    • Piston skirt scuffing and cylinder liner wear and scuffing
    • Exhaust valve face and seat deposits
    • Piston pin and slipper bushing wear
    • Overall valve train and bearing wear levels

A word of Caution

There are hundreds of commercial crankcase oils marketed today. Obviously, engine manufacturers or users cannot completely evaluate the numerous commercial oils. The selection of a suitable lubricant in consultation with a reliable oil supplier and observance of his oil drain recommendations (based on used oil samples analysis and experience), and provide filter maintenance, will provide the best assurance of satisfactory oil performance.

It should be noted that lube oil manufactures may reformulate and oil while maintaining the American Petroleum Institute classification, or may reformulate to a new API classification and continue the brand name designation. For example, SE oils being reformulated to SF letter code classification may perform differently after this reformulation. A close working relationship with the lube oil manufacturer should be maintained so that any reformulation can be reviewed and a decision made as to its effect on continued satisfactory performance.

Detroit Diesel® Used Lube Oil Analysis Program

The Detroit Diesel® lube oil analysis program is recommended for monitoring the condition of the crankcase oil in all engines.

Primarily, used lube oil analyses indicate the condition of the oil but not necessarily the condition of the engine. Never tear down an engine based solely on the analysis results obtained from a single used oil sample. However, the condition of the engine should be investigated using conventional mechanical and/or electronic diagnostic instruments. Frequently, visual inspections are all that is required to detect problem areas related to engine wear. It is also prudent to obtain another oil sample from the suspected distressed unit for analysis.

Abnormal concentrations of some contaminants such as diesel fuel, coolant, road salt, or airborne dirt cannot be tolerated for prolonged periods. Their presence will be reflected in accelerated engine wear, which can result in less than optimum engine life. The oil should be changed immediately if any contamination is present in concentrations exceeding the warning limits.

6SA314 8901

Oil Level Check:
The correct way to check the oil is to shut off the engine with the bus sitting on a level spot. Let it sit for a couple of hours and drain all back into the oil pan, then check it. If you check it after just shutting the engine off it will check about 1 qt.. low up to about 15 minutes or so. (Depends on the temperature of the engine oil, the hotter the faster it will flow back.) If the vehicle is not level use common sense and if the nose is high, the oil will show over what it really is. If the nose is low the oil will show lower than what it really is. If the left side is lower than the right side it will read lower and visa Versa.  The best time to check the oil is just before you start it while it is level. After you start it you can test the level and you will find that it will show approximately 1 gallon low while running if it was at the full mark before you started it.

It is a good thing to check your dipstick just after an oil change after you have installed the correct amount of oil that the bus manufacturer recommends to confirm that the dipstick has the correct markings on it. It is not unusual to find that the dipstick has been replaced with the wrong one or is poorly marked. Install the correct amount of oil to include that which is in the filter, start the engine let it run until it reaches 180 degrees, shut the engine off and wait at least 2 hours. 

Russ Long’s Popular Oils List:

The most popular and correct engine oils for the two-stroke Detroit Diesel engines (but not limited) to are the following:

Detroit Diesel PowerGuard 40wt
Mobil Delvac 1240
Chevron Delo 100
Miles Labs HD Diesel 40wt
Shell Rotella T

Detroit’s PowerGuard is Mobil Delvac 1240 which is private-labeled.

Most of these oils are not on the shelf at your local auto parts store, they must be ordered. You can find distributors by using the “dealer locator” tab on each manufacturer’s website.

It can take anywhere from five – nine gallons of oil for an oil change,
depending on the bus make/model, engine and oil pan size. If you’re having a shop change the oil for you, always bring your own, correct oil – don’t rely on the shop to have it.

Learn a little more about the history of Detriot Diesel here.

Having issues with your Detroit Diesel Engine? Check out our troubleshooting guide.

Bus Nut Service Center Map

As a community, we want to make sure all of our members are taken care of. In our journeys, there is a likelihood that we will require the services of a service center to take on tasks from the simple to the extreme. We created this map as an easy reference in the event that you were to need assistance while on the road. We also thank those of you that have given you input on the shops and service centers that have worked on your vehicles. This map is 100% user feedback so we feel the results are only that of great service to those when they needed it the most.

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