The right oil grade and quality is probably the single most important factor in car maintenance. If you want your engine to last as long as possible, the absolutely most important thing you have to do is to change the oils and fluids regularly. Cheap engine oil, neglected oil changes and the wrong viscosity is going to drastically decrease the longevity and reliability of any combustion engine. Why is it so important? Because the engine oil lubricates all crucial parts inside the aggregate and guarantees a smooth operation. Any kind of deviation like low or dirty oil can cause premature wear and breakdown quickly. Dirty oil is the enemy of lubrication and long motor life. Sludge builds up, engine oil gets contaminated with fuel, acids and water accumulate which leads to accelerated wear and corrosion, oil delivery pipes get clogged up, oil starts to burn up in the combustion and clogs up exhausts, valves, catalytic converters etc. The list of possible failures goes on.
Single Grade Oils
Historically, there were only single-grade oils available that were refined from crude fossil oils. They only featured one "thickness" or viscosity number. For example 30W. The number was followed by a "W" standing for winter. It was the oil’s viscosity rating from cold when the engine was first started in the morning. As the oil heats up, it becomes thinner and loses its viscosity. Too thin oil can cause a break in the oil film and also leads to excessive oil burn. It was almost impossible to balance between sufficient lubrication at a cold start and during a heated-up engine. But as technology advanced, multi-grade oils were introduced, which feature two types of viscosities depending on their temperature. They employ Viscosity Index Improvers or "Polymers". Modern engine oils have a different viscosity when cold and when hot allowing the engine to run optimally at all temperatures. A second number was added to the "W" stating the warm viscosity number. For example 10W40. The 10W stands for the cold rating and the 40 indicates the warm flow rate.
Today's engines have tighter manufacturing tolerances and therefore require a thinner oil. Variable valve timing, valve lifters and other oil-actuated systems need to get into all the small passageways and require a thinner oil at a cold start. Last but not least emissions regulations get even more stricter by the year and engines need to become ever more efficient. Modern engines therefore often feature 0W or 5W cold viscosity ratings but maintain the hot rating of 30 or 40. This is because modern engines produce more power and more heat and require a stable and viscous oil for best performance and stability. The epitome, for now, is the BMW E46 M3 and the Maserati 3200 requiring a fully synthetic 10W60 to guarantee a very resistant viscosity oil due to the massive engine power. This oil is very thin at the start but the number 60 ensures that the oil never becomes too thin to damage bearings when hot.
Evaluating the Cold Oil Grade
In order to determine if the current oil is sufficiently lubricating the engine in cold operation, open the bonnet, start the engine and watch the oil pressure gauge and/or the oil pressure warning light. During a cold start, the engine should be able to establish oil pressure fairly quickly within a few seconds. When I look at the oil pressure gauge in my Mercedes, it rises to 3 bar after around 6-7 seconds. If there is no gauge, watch the oil pressure/level warning light and how long it takes to turn off. High viscosity oil prevents oil pressure from building rapidly. If it takes more than a few seconds or if the pressure is low, the cold viscosity rating of the oil might be too thick for the climate. While monitoring the oil pressure, also listen for suspicious rattling and ticking sounds coming from the engine specifically from the valve lifters. If the car has hydraulic valve lifters a cold and thick oil might not be able to get into the small passageways of the lifter and cause rattling due to the gap between the camshaft and the valve stems.
Evaluating the Hot Oil Grade
When the engine has reached its operating temperature of around 70-80 °C, continue monitoring the oil pressure gauge and listen for ratting. When revving the engine, the pressure should rise quickly. There shouldn't be any rattling noises caused by too-thin engine oil. Also, evaluate the smoke coming from the tailpipe. If there is a blueish-white cloud exiting it's a sign of oil burn, which can be a cause of too thin oil (or a worn/damaged engine). An old engine wears out and clearances get bigger, which leads to more rattle and more oil consumption. The standard oil might not be sufficient anymore to fill the gaps, seal the engine and prevent noises. A too thick oil, on the other hand, might not establish sufficient oil pressure.
Choosing the Right Viscosity
First of all, you should consult your owner manual and check what engine oil viscosities are recommended to establish the baseline. You will likely come across a chart that shows the different oil indexes related to climate temperatures. Choose the one that fits your climate temperature range the best. From there, you can take a decision if you want to bump up or down the cold or hot rating by 10 or 20 points depending on if the engine has received performance upgrades, experiences rattle during startup or has run many miles already. With an engine that has run +100'000 km, I'd recommend bumping up the hot rating 10 points to give it a better and more resilient oil to work with. A slightly thicker oil will prevent further wear.
Why classic cars often run 20W50
Classic cars, having aged and lived with varying owners, various climates, differing driving styles, maintenance procedures and perhaps even having the engine rebuilt will have a unique working environment for their oil. Tolerances get bigger with wear and tear, altering gaps, a build-up of soot and oil sludge changes the diameter of galleries and the car may have spent the first decade of its life enduring short stop and start journeys. Many classic car owners, therefore, upgrade the oil viscosity from a 10W40 or 15W40 to a 20W50 which provides more stability. Since many classics are only driven in the summer, a higher cold rating is unproblematic. 20W50 is often available in car shops and is labelled as "classic car" engine oil. It is a semi-synthetic oil and is best suited for a variety of classic cars. Often, racers such as the Bimmer fans use this oil too for street and track racing because of its high protection properties and high-temperature stability. For winter temperatures a different viscosity is recommended to provide better protection during startup.
Many classic car engines have a pushrod valvetrain which requires additional lubrication. In order to prevent premature cam and lifter wear, a sufficient amount of anti-wear Zinc additives are required. 40 years ago this wasn't a problem but due to environmental reasons, the quantity of an additive called ZDDP (Zinc Dialkyl Dithiophosphate) has been reduced significantly in modern oils. Zinc helps grease engine parts and control oxidation and corrosion. The ZDDP content was lowered in conventional motor oil to prevent the possible fouling of the catalytic converters. Levels for ZDDP in oils used to be 1,200 to 1,400 ppm. Since the early nineties, manufacturers have been reducing this number. The latest automotive oils contain only around 800-900 ppm. Most engine and component manufacturers recommend a zinc content of more than 1,200 ppm for break-in, and many will void warranties if this minimum is not found in the oil sample that is supplied when returning broken parts for warranty. There are many Zinc oil additives to choose from that can be added to the engine oil.
As you can see, there are many considerations when choosing the right engine oil. There are no "universal fits all" oils, but a host of different factors need to be taken into account. For a classic car, I think it's best to always run a more viscous oil and bump up the SAE rating by at least 10 points to guarantee maximum longevity and protection. A more viscous oil does reduce efficiency by a few percentage points though. Just don't go too light and avoid 0W oils which are only suited for extremely cold climates where it gets below -20 °C. Formula one cars run extremely light oils to gain every horsepower they can but these high-performance engines need a rebuild after every race because they are worn out.
If you want to learn more about classic car maintenance and repairs as well as tips and trick, consider the book "The Ultimate Classic Car Guide". It will give you everything to take care of your own classic car without having expensive garage bills.
Nils is a Swiss-German engineer who is obsessed with old cars and engines. He is the author of "The Ultimate Classic Car Guide - How to Buy, Maintain & Repair Classic Cars" and the founder of EVC. My passion has always been with old cars and everything that has wheels and an engine.