This is a complicated topic but I’ll try my best to summarize it as best as I could. The info here are from posts/papers/discussions that I read and are scattered across the internet but I'll be including a link to one discussions that gives you the detailed version of what I posted.
The best engine oil for your car depends on where you live, your car’s current mileage, your car’s service history, your engine’s current condition, your operating environment, mostly highway or city driving, and your desired fuel economy.
Bottom line - your car’s manufacturer recommended oil represents a compromise that may not always be in your best interest.More on this later.
Engine oil is a lubricant that (1) protects your engine from friction, (2) absorbs heat from the combustion chambers of your engine, (3) channels away the byproducts of combustion like silica, and (4) inhibits corrosion.
As oil heats up, it gets thinners and flows more readily. However, when the oil gets too thin, it does not stay on the surface of the metal long enough to act as a proper lubricant.
The goal of oil manufacturers is to prevent the oil from getting too thin when hot but thin enough to flow when very cold. Due to the dual nature of the problem, oil operating at cold and hot temperatures, designers came up with a dual grading system that accounts for both extremes.
Engine designers typically design the engine to operate ideally at 212F (100 degree Celsius of oil and water temperature) and the oil to have a weight of 10 at that temperature. The ideal engine oil weight of 10 is why you need to warm up your engine before driving to give the oil enough time to get hot. In some engines like most modern BMWs, the engines have a very high compression ratio that benefits from immediate SLOW driving upon startup.
When your car gets to 212F, the engine oil weight will be at exactly 10. Wear to your engine actually starts when the oil is too thick to lubricate the engine internals upon startup and gets diverted into the engine sump bypassing the engine when you step on the gas pedal.
Grades
The Society of Automotive Engineer (SAE) established viscosity gradings (0, 5, 10, 15, 20, 25, 30, 40, 50 or 60) that are used to categorize oil. The numbers are marked on a typical quart of oil, e.g. 10W-30.
What does the 10W-30 number really mean?
The number before the W is the cold viscosity rating and the number after is the hot viscosity rating. A 10W-30 oil will behave like a 10 grade oil when cold and like a 30 grade oil when hot. In a cold climate, you want a very low number before the “W” and in a hot climate, you want a high number after the “W”.
In a country like Nigeria, you don’t really care about the number before the W because the weather does not get cold enough.
What is sludge?
Sludge is what happens in your engine when the oil additives and chemicals break down and turns the oil into gunk or tar-like substance (black custard). It can occur due to prolonged oil change intervals or due to high engine heat exposure. Sludge can/will cause your engine to seize from oil starvation.
Some engines are more prone to sludge no matter your oil change interval, e.g., Toyota and Audi engines. Here’s a Class Action Suit that has already been settle against Toyota and Lexus for sludge problems:
http://www.oilgelsettlement.com/Documents/detailed_notice.pdfIn response to sludge problems, Audi/VW changed their service interval to 10,000 miles on the 1.8T engines.
Do I want greater fuel economy or greater engine lifespan?
If you only care about fuel economy, then you would want to buy the thinnest oil possible because there will be less restriction to oil flow in the engine, which is why manufacturers are now gravitating towards thinners oils like 5W-20 from 10W-30.
If you only care about greater engine lifespan, then you would want the thickest oil possible because it offers you more protection from severe operations and greater protection from heat. Thicker oil can readily absorb and retain heat from the engine but if you’re in a cold climate, this might be a problem because it might be too thick to flow through the engine when you first start the car.
Summary of the benefits of different types of oil grades
0W-30, 0W-40, 5W-40 (Fully synthetic)
1. Fuel economy
2. Performance and power
3. Protection from deposit build-up
4. Great in very cold climates
5W-30, 10W-40, 15W-40 (Semi-synthetic)
1. Better engine protection
2. Great engine protection within first 10 minutes of starting the engine
3. Three times (3X) better at reducing engine wear
4. Increased oil change intervals
10W-40, 15W-40
1. Need to change oil more often because the additives wear out and the composition of the oil changes which is not true for synthetic oils.
In Nigeria, I would say the best oil would be something around 10W-40 and 20W-50
