20W-50 Engine Oil Comparison

I’ve recently discovered that there’s a lot more things to consider when choosing the right engine oil for a classic car than just the grade, e.g. 20W-50.  These subtle differences can make a significant difference to not only performance, but more importantly the life of the moving parts of the engine –  a major consideration for me, since my classic car is a daily drive.

One key aspect that I’ve discovered was related to the amount of Zinc DialkylDithioPhosphate (ZDDP) present in the oil.  As a simple summary, this helps the lubrication of moving parts that rub together, and was an essential factor in oils back in the days.  However, this has been reduced over time, mainly due to the adverse effect of ZDDP on catalytic converters in later cars, but it turns out this is understood by many “quality” oil producers, who ensure this is still added to protect classic cars.

Duckhams have written an excellent article about ZDDP, which can be found here, and I have compiled a table of popular engine oils, listed in order of my personal favourites.  My consideration has been the balance between Viscosity (VI), Performance (API), ZDDP (measured in parts-per-million),  the level of Detergent, and price (not shown, as this can vary).

If you want to know more about the API standards, please  click here for the section of the API website detailing different oil categories, and here for an excellent news article on the Rymax Lubricants website explaining how the API specification works.

I hope this is useful, and wonder if you’ll share my surprise when you realise that the “quality” of some very well-known engine oil brands may not quite be as you had thought!

DetergentData Sheet
Millers Classic Pistoneeze 20w50133SJ1130High
Fuzz Townshend Classic Oils Heritage 20W50131SL/CH1300High
Duckhams Classic Q20W-50135SJ/CE1300Med.
Penrite Classic 20W-50125SL/CF1080Med.
Castrol Classic XL 20W-50124SF/CC800Low
Morris Lubricants
Golden Film
Motor Oil
Comma Classic 20W50