Which Engine Oil Should I Use in My Old Car?

QUESTION:  Hi John, I highly respect your opinion on anything relating to cars. A fellow who has an original owner's manual for a 1964 EH Holden says it recommends to use an SAE 20 or a 10W30 engine oil. My local auto accessories store owner who has a number of classic and vintage vehicles says 10W30 oils weren't around then so I don't know who to believe. He has suggested I go with Penrite's HPR30 (20W60).

Penrite's lube guide recommend the 20W60 oil, Valvoline recommend a 10W30, Castrol recommend a 15W40 or a 20W50, Nulon recommend a 15W40 and Chief Oil recommend a 10W30.

Penrite seem to be at odds with the others, it's a very thick oil but it is full zinc. I'd be interested to know your opinion and what way you'd go. Thanks, Chris


Part of this answer really depends on what condition the engine is in. Obviously as engines wear, the clearances between the close-fitting mechanical parts increases, and - up to a point - this can be compensated for by using a thicker engine oil.

How Oil Grades Work

The numbers work like this: For 15W30, you need to separate them into '15W' and '30'. The 15W relates to when the oil is cold (think: winter) and the 30 relates to the oil at about 100 degrees C.

But let's back up a second. Obviously, most liquids get thinner as they get hotter - like honey, or maple syrup, for example. Oil manufacturers use chemical engineering tricks to make oil thinner as it gets colder (or at least, to make it thicken up less as it cools). Obviously the thinner the oil is when it is cold, the quicker it gets pumped around the engine upon start-up. This is good because it reduces engine wear.

What this means is that a 5W30 oil and a 15W30 oil perform exactly the same at 100 degrees C, but the 5W is thinner when it's cold.

Back in the '60s

Back in 1964, engine oils were mineral-based. As in, derived from crude oil. There were limits on the amount of chemical engineering possible, and the engines were designed to work with - probably - a 40-grade oil. As in a [number]W40 oil. The clearances in the bearings were designed just so - so that a very thin film remained between the rotating parts once they had expanded to their normal operating temperature.

As the engine wears, you might elect to go up to a thicker [number]W50 oil so that a thicker film remains in the increased space between the more worn bearings.

Since the advent of synthetic oils, more chemical engineering tweaks have been possible. Thinner oils are routinely used - today 0W20 oils are used on some cars. The advantage is they get pumped around very quickly, and they reduce internal frictional losses in the engine - but they require much tighter operating tolerances between the bits that count.

Depending on the condition of the engine I think it would happily receive 20W50 mineral oil for the rest of its days if it is a bit worn, or 15W40 if it has been rebuilt in the past 10-15 years and not used much. Part of the confusion here is that the numbers I am talking about are SAE oil grades. The Penrite 20W60 is an API SG/CD grade oil. This is a different oil rating system - you'll note in the fine print that it's claimed to be compatible with SAE grades 20W40 or 20W50. (API is American Petroleum Institute; SAE stands for the Society of Automotive Engineers.)

Valvoline does both SAE grades today (see below). 15W40 is API SL/CF, and 20W50 is API SJ/CF. 

Valvoline XLD 15W40 (click to enlarge)

Valvoline XLD 20W50 (click to enlarge)

Hello, World!

API Ratings

In these API ratings, 'S' stands for 'spark' (meaning spark ignition, or petrol, engine) while 'C' stands for 'compression' (meaning compression ignition, or diesel, engine).

API ratings were introduced progressively, over time:

API ratings system (click to enlarge)

Oils rated SA to SH are generally no longer manufactured. So, what they did essentially was make the SJ oil backward-compatible with SA-SH grades. SL and SM are also backward-compatible. The Penrite Classic Light 20W60 is API SG compatible.

Here's some detail about which API grades are available today.

API oil grades - click to enlarge

Contrary to popular belief, oils ARE oils - meaning any reputable brand of oil made to one of those grades will be fine. Every oil maker can give you 100 reasons why theirs are the best. The only thing that matters is what the specification is.

If the engine is a bit clattery with 15W40 on board, change to 20W50 next time.  And remember to change every 3-6 months and/or 5000km - whichever comes first.