Finding the right oil for your classic car can be a minefield. It is important to narrow down the field, for there are many variables and certain vital considerations when choosing suitable oils for both engines and transmissions.
It’s most important when considering your oil choice to know the history of the engine. What oil was recommended for it when it was new and what has been used in it since, especially in recent years?
Almost equally important is, what’s going to happen in the future? Is your car going to be thrashed on rallies, track days or races, or is it going to potter gently to the occasional show? Is it going to cover 5000 miles a year, or 500?
Until the advent of multigrade oils in the 1960s, car manufacturers used monograde oils – and specified an oil change every month or 2000 miles, with different grades of oil required for different seasons or climates. Engines were frequently not fitted with oil filters or, if they were, they just had bypass filters or simple gauzes on the oil pick-up. There was no detergent in the oil and it was expected that the engine would need the cylinder head removed for a de-coke and the sump removed for a clean-out every year or at least every 10,000 miles.
(That’s one reason Ford stayed with the side valve engine for so long – a de-coke service was far simpler and cheaper than on an overhead valve engine, and most owner-drivers could do the job themselves.)
Engine life was much shorter than we expect now; bore wear due to poor oil film, inadequate protection when cold and carbon particles everywhere meant that it was normal to ire-ring the pistons at 30,000 miles, with a rebore and crank grind at 50,000.
It is still possible to buy cheap non-detergent monograde oils like those original 1960s ones, but are you sure that’s the sort of engine life you want?
Multigrade oils brought a huge advance, the SAE 20W/50 giving the flow and penetration of an SAE 20 monograde at low temperatures on start-up while maintaining the protection that an SAE 50 monograde would give at high loads and temperatures. It’s all done by the use of polymers that are coiled up at low temperatures but unwind at higher ones.
Detergent additives help to mop up impurities that would otherwise be deposited around the engine to impede flow and increase wear, carrying them to the filter where they can be removed safely from the system. Anti-oxidants help the oil to last longer, emulsifiers soak up condensation, while the purposes of anti-wear additives (usually zinc) and corrosion inhibitors are self-explanatory.
All are blended into modern oils and make a major contribution to the 200,000-plus miles life we now consider normal for engines. Indeed, modern big truck engines can go 50,000 miles between oil changes and do a million miles between rebuilds.
Synthetic oils are theoretically even better: they are inherently less prone to oxidizing at high temperatures and have stable viscosity, high film strength and excellent low temperature flow, and leave minimal deposits. However, that doesn’t always make them ideal for your engine.
If your engine has been run for years on non-detergent oils, a sudden change to detergent oil could scour so much crud off the surfaces that it blocks narrow oilways, resulting in starved bearings and rapid engine failure. Stripping and cleaning the engine before changing is the only safe answer. Once done, you can safely use detergent oils in any engine.
Abandoning monogrades is a more sensitive issue. Older engines were designed to work with the oil that was available when they were new – and specialists now produce non-detergent monogrades that have all the other beneficial additives. Multigrade oils are not recommended for roller-bearing crankshafts.
Unfortunately, being better at penetrating and flowing to provide lubrication means low-viscosity oils are better at finding their way out of the engine, whether it’s past the piston rings and valve guides to be burnt off in the combustion chambers or past the bearings and oil seals to drip onto the floor or even contaminate the clutch linings.
Once engine wear is added to the picture, the problem is even more marked, as it increases tolerances and wastage. For worn engines, it’s vital to use an oil at least as thick as originally specified, if not thicker. The 20W/50 multigrade (original spec for many 1970s engines) is now often marketed for worn engines that would have used 10W/40 oil when they were new. Penrite offers 25W/70 and 40W/70 oils, giving better protection for worn engines designed to take at least 20W/50 when new, though it’s not cheap; if your oil consumption is on the high side, you may prefer a cheaper (but still good quality) 20W/50.
Synthetic oils are invariably thinner, so only suitable for a classic car engine if it has been rebuilt to closer tolerances and with more effective oil seals and gaskets: in this case synthetics are ideal.
With a quality modern multigrade oil in an engine that has good oil and air filtration, especially if it is not unduly worn, you can safely increase oil/filter change intervals to 5000 miles or more: 20,000 with synthetic oil. Keep an eye on the level and color of the oil – if it’s getting black or sludgy before then, change it; otherwise, there’s no need.
Modern transmission oils can harm older (pre-1960s) transmissions where soft yellow metals are used. For these, specialist oils to API GL1 must be used.
It’s the Extreme Pressure sulphur/phosphorus additives that do the harm; these are present to a small extent in GL4 gear oil and to a greater extent in GL5 lubes. Thus GL5s offer greater protection for the gears, but will attack yellow metals and may also contain friction modifiers that can cause slipping in overdrives.
Multigrade transmission oils are excellent if oil seals are up to the job of keeping it in – otherwise, you will have to stick with original-spec monogrades.