Now that you understand viscosity, it will be easy to understand the difference between the grades or weights of oil. For the most part, there are two types: straight weight or multiweight (or multigrade).

Most street engines operate in a wide temperature range. Therefore, the oil must be able to adapt when it is extremely cold or brutally hot. For instance, the oil must flow sufficiently during cold-start conditions when the engine has been sitting overnight in freezing temperatures, yet it must not thin out too much once the engine reaches operating temperature.

Multigrade oils bridge this gap wonderfully because they have the ability to change viscosity with temperature. To understand, let's look at a typical multigrade oil, such as a 5W30.

So what do those numbers and letters mean? Simply stated, the "5" relates to the viscosity at ambient temperature (normally 100 F), the "W" stands for Winter, meaning the oil is suitable for winter use, and the 30 is the viscosity of the oil at the second test temperature (normally 212 F). In other words, when the engine is cold, the oil acts like a 5 weight and it flows easily. As the temperature gets to 212, the oil thickens and acts like a 30 weight.But there's a catch. For years we've been told that oil thins out when it gets hot. This theory tells us that the opposite is actually occurring. So which is right?

Here's the deal. Multigrade oils use polymers, which are added to a lightweight base stock. The polymers react and prevent the oil from thinning much once the oil temperature goes up, so the oil does not show a drop in viscosity. This is not the case with straight-weight oils.

It's important to remember that viscosity ratings are given at a certain temperature, and the oil changes viscosity as temperature changes. Therefore, you cannot directly compare straight-weight oil to a multigrade unless you know the exact operating temperature of the oil and its corresponding viscosity. Straight-weight oils become thinner as they are heated, so viscosity does indeed drop.

Oil Level

Some folks feel the safe bet is to overfill the oil pan. This, undoubtedly, is based on the "more is better" theory, but overfilling the oil pan is not a good idea. Raising the level in the pan beyond what the factory recommends will most likely cause the oil to surround the lower half of the crankshaft, causing excessive drag and windage as the crank rotates. Excess oil will also be splashed on the cylinder walls, increasing the chance that the oil will get past the rings where it will be burned in the combustion chambers. This alone can lead to detonation and smoking--it also kills power and economy. If you feel the need to run additional oil for added protection, install a larger-capacity pan or an Accusump system.

When it comes to road racing, go with the recommended level of oil in the pan. If you want more capacity, install a remote cooler along with the aforementioned oil accumulator/Accusump.

During extended severe or road-racing use, the engine and the oil will be taxed much harder than in drag racing or normal street driving. Having sufficient oil in the engine will do a number of things to improve performance. Know that the oil helps the engine dissipate heat, therefore sufficient oil supply will help keep engine temp down. Also, a low level increases the chance of starvation in cornering and braking. For this last reason, we highly recommend an aftermarket oil pan with proper baffling.

One trick used by drag racers (especially when the stock pan is used) is to reduce the oil level in the pan. Lowering the oil level keeps the oil off the crank, thereby reducing drag and releasing horsepower. Note: This is not recommended with modular engines because they require more oil at the top end of the engine to lube the camshafts. Lowering the oil level can starve the cams, leading to damage. In addition, the crank is mounted higher in the block when compared to Windsor engines, so there is less benefit from having less oil in the pan.

Filtration And Oil Change
Ever notice that while you pour in that clean, golden oil it never comes out that way? More often than not, it comes out black, and even its smell changes. Sometimes it will reek of fuel, other times it just smells nasty. Well, believe it or not, this is good. By the oil changing color, you know that it is doing its job of ridding the engine of contaminants, which can be anything from small particles of dirt to metal fragments picked up internally. The oil carries these particles until it is captured by the oil filter or drained from the engine.

Despite the best efforts in filtration, some of these particles will make their way into critical components. This leads to increased wear of the bearings and the cylinder walls. For this reason, you must pay close attention to your oil. Often, it tells a story. You can further inspect the oil by cutting open the filter and/or having a lab analyze the oil.

The oil will contain contaminants from combustion, which leaks past the rings. They can break down the oil and shorten its life, too. This is common in racing engines, due to richer mixtures.

For these reasons, we strongly suggest following the manufacturer's recommendations for changing the oil, or changing it more often if you are a hard driver. In racing, you should use your own discretion or that of your engine builder. Some racers hold out, but a few quarts of oil and a filter is cheap insurance. For that reason, I swap out the oil in my own race car every other weekend. That's usually about every 12-15 runs.

Lastly, there are a variety of oil filter types available. Most will be the spin-on type, although some use a reusable filter and can be disassembled, which allows you to inspect the filter. Routine inspections can reveal metal in the oil, and this helps to catch a problem before it ruins your engine.