Question: My owners manual specifies 5w20 oil. Do I really need to use 5w20 oil and why did my 2000 model year vehicle require a 5w30 oil, while the exact same engine in my 2001 model year vehicle requires 5w20 oil?
Answer: No. You do not need to use a 5w20 oil. The only reason 5w20 was specified for your engine is to increase the CAFE (Corporate Average Fuel Economy) reported to the Federal Government. CAFE is the combined average fuel economy of all of a vehicle manufacturer's product line. Minimum CAFE levels are specified by the Federal Government. In order for a vehicle manufacturer to continue selling profitable large trucks and SUV's, which typically have poor fuel mileage, and still meet mandated CAFE requirements, they must also sell smaller cars which have much better fuel economy ratings to offset the poor fuel economy ratings of the larger vehicles.
For model year 2001, the change to 5w20 oil will allow a vehicle manufacturer's overall CAFE to decrease by a very small amount, typically in the tenths of a mile per gallon range. 5w20 oil is a lighter viscosity than a 5w30 oil and therefore has less internal engine frictional losses, or less drag on the crankshaft, pistons and valve-train. This decrease in frictional power loss promotes increased fuel economy. This increased fuel economy is virtually undetectable to the average consumer without the use of specialized engine monitoring and testing equipment when compared to a 5w30, 10w30 or a 0w30 viscosity motor oil.
Question: What are the negative aspects of using a 5w20 oil?
Answer: 5w20 oil has less film and shear strength than a 5w30, 10w30 or a 0w30 motor oil. This can lead to increased engine wear under today's demanding heat and high-stress engine performance conditions.
Question: Is the above true of synthetics as well?
Answer: Yes and no. Depends upon what you're comparing. A 5w20 synthetic oil will likely have less film and shear strength than a synthetic 5w30, 10w30 or 0w30. However, if you compare that same synthetic 5w20 motor oil to a petroleum 5w30, or 10w30, it will likely have better film and shear strength.
Obviously, this means it will certainly have better film and shear strength than a petroleum 5w20 oil. So, if you'd feel more comfortable sticking with the manufacturer recommended 5w20, go with a quality synthetic oil.
Question: Don't I have to use a 5w20 oil to maintain my factory warranty if my manual recommends a 5w20 oil?
Answer: Absolutely not. Vehicle manufacturers recommend using motor oils meeting certain viscosity grades and American Petroleum Institute service requirements. Whether the motor oil is a 5w20, 5w30, 10w30 or 0w30 or even a synthetic vs. a petroleum-based oil will not affect warranty coverage. The manufacturer is required to cover all equipment failures it would normally cover as long as the oil meets API service requirements and specifications and was not the cause of the failure.
In addition, the federally mandated Magnuson - Moss Warranty Improvement Act states that a manufacturer may not require a specific brand of aftermarket product unless it is provided free of charge. If your dealership continues to tell you that you need to use 5w20 oil to maintain your warranty, then ask them to put it in writing. If they indicate you must use a particular brand of oil to maintain your warranty, tell them you expect it to be provided free of charge.
Their position is inaccurate and, in fact, violates existing U.S. legislation. Additionally, if there is ever a question of whether or not a particular oil was the cause of an engine failure make sure to get a sample of the used oil in a bottle, typically 6 oz. minimum. That will provide enough fluid to send oil to 2 independent testing labs for analysis. Remember, being a knowledgeable and informed consumer is your best defense against being taken advantage of by a car dealership service center.
NOTE: If you've purchased an extended warranty, the above may not apply. Extended warranties are not necessarily bound by the same regulations as factory warranties are.