Technical Dept.

Tech # 5

All Cars

Subject: Engine Oil, When to change, What to use

With the vast array of oils available and the endless hype by auto manufactures and oil companies it is difficult to know what to do and when to do it. The following is my recommendation.

Lets start with some of the misleading hype from many auto manufactures. Many of them tell you that you should use 5w / 30  or similar oil and change every 7000 miles.

Lets look at the number first. To better understand the viscosity number of oil you can think of the oil as marbles. A little marble, # 5 will fit into a thin opening between two objects and if you move the two objects in opposite directions the # 5 marbles will roll and keep the two surfaces from rubbing each other. Now if you took all the # 5 marbles out and installed just # 30 Large marbles. They would be too large to fit between the two surfaces and the surfaces would rub against each other and quickly destroy the surfaces. ( IN SECONDS). Suppose you have two surfaces that have enough clearance to accept a # 30 marble and the two surfaces will roll the # 30 marbles just fine. Now you remove the # 30 marbles and install just # 5 marbles. If the two surfaces move close to each other they will contact the # 5 marbles and roll OK but, the two surfaces have a lot of room to move. This movement can allow the two surfaces to crash into each other, thus squeezing out the # 5 marbles and allowing the two surfaces to rub and self destruct. Thus, a multi grade oil like 5w30 or 10w40 will meet the standards of a wide range of clearances. However, that is not the whole story. Temperature is a major factor in this. Viscosity tries to decrease as temperature increases. Meaning, a # 30 marble when too hot will become a # 5 or less. The ability of an oil to maintain it's viscosity as temperature increases is called it's viscosity index. Penn. crude oil has a naturally high viscosity index meaning it will try to keep it's viscosity when hot. Chemicals and treatments can boost the viscosity index of an oil. STP is a brand of a viscosity index booster among others.  An engine that can use 5w30 oil will need to raise the viscosity numbers as it ages. The 6 to 7000 mile oil change is not a savings for the car owner. Not in the long run. You are saving a few pennies by not changing the oil but it IS costing you engine ware, Unnecessary Engine Ware. It's  not that oil wares out, it's the oil gets contaminated and is in a continuous process of being distilled and diluted by fuel and countless chemicals in the gas and in the oil. Even though you may get away with a 7000 mile oil change at first, you won't get away with it long. Always change the filter when you change oil. If your car is still in warranty I recommend that you use the factory recommended  oil filter and even the recommended oil, just change it every 3000 to 4000 miles with the filter. Especially if they say change every 6000 or 7000 miles or if they say change filter every other time.

Do's and Don'ts 

Do change oil and filter every 3000 to 4000 miles under normal driving conditions.

Do use factory recommended oil and filters while in warranty.

Do raise the viscosity number in hot climate and lower the number in very cold climate conditions.

Do warm the engine up at a fast idle in cold climate conditions before driving.

Do keep your crankcase vent system checked. It can have a effect on your lubrication system.

Don't ever add a different brand of oil to oil in your engine unless you are sure you are going to change oil and filter in a day or so.

Don't think you can drive to the nearest service station or even the next exit if your oil light comes on while driving. Major damage starts within seconds of when the light comes on. 

Don't buy a brand of oil or filter Just because it is cheaper.

Don't buy a brand of oil or filter Just because it is the most expensive. Do at least a little research.

Don't disable any emission control devices in an effort to gain horse power. The main reason is that you will NOT gain any horse power by disabling any emission control devise.

Don't use Castrol Syntec oil in a fresh rebuilt engine. And according to Castrol, don't use their oil if the date code on the container is over three years old and DON'T use their oil in a modified engine.


your comments or suggestions.

Return to tech page.

This site was brought to you by

HMF Inc.

PO Box 246 Smyrna, TN  37167