What’s important when choosing engine oil for a classic car engine?

Well, it’s a discussion that pops up every now and then – all over again and I got numerous mails about what kind of engine oil I use for my classic cars and why. Most important criteria for the selection of the best oil for a classic car are viscosity, conventional or synthetic and vital additives.
In so many ways, modern car engines are a lot different than the ones of classic cars. Metal-to-metal production tolerances, the way those engines are revving, not to mention the not so rare rope or cork seals. Possible fouling of the catalytic converters due to zinc and phosphate led to a reduction of these vital additives for classic car engines
 
My advice: either use high zinc and phosphate concentration oil like the ones in the following list – or – if you buy oil randomly, add some zinc & phosphorus additives.

Viscosity

Engine start-up is where the most wear occurs, that’s for sure. So it might sound obvious to use the lightest weight oil. But, that also depends on the design of the engine. That might be the case for more modern engines, but if we’re talking dry sump Porsche 911 engines, 20W50 is the right choice. Engineers from Porsche, which I had the chance to talk to, recommended 20W50 viscosity for all early 911s. I’ve tried a lot of different kinds of oil in different cars and with the 20W50 oils I’ve tried, I was always in the ideal oil temperature and therefore engine temperature range. 

Synthetic Oil

For some people knowing they’ve got the best and most expensive oil is a relief. Always consider the loose tolerances of older engines. I’ve heard of people at LA’s cars and coffee that their cars leaked oil like the Titanic when they first tried synthetic. It’s much higher in price, so it might make sense to use conventional oil and change it more regularly. 

ZDDP

 
Zinc Dialkyl Dithiophosphate (ZDDP) is an anti-wear additive that helps lubricate engines and also helps to control oxidation and corrosion. The American Petroleum Institute (API) published guidelines to lower zinc content in oil and all manufacturers followed the request. To give you an idea, the levels of ZDDP used to be 1200-1400 ppm (parts per million). For almost 30 yrs already, most oils are around 800-900 ppm. Thats way to low for a vintage car engine. You might not realise the difference in a day or week or month, but over the years, your engine will age prematurely. Especially if you have your engine rebuilt, the lack of proper lubrication can be fatal. Make sure you add ZDDP or use racing oil (more phosphorous) for that purpose. After the break-in, you might go back from racing oil to normal oil. General rule: make sure you have at least 1200 ppm for break-in.

ZDDP additives

So if you’re buying oil from the shelf, which is still better in terms of the quality than 30yrs ago, but please make sure you add some ZDDP, a 4-ounce bottle to a five-quart oil change.

ZDDPlus Engine Oil Additives & Phosphorus, highly recommended ZDDP additive

Some cool tools for your next oil & filter change can be found here.

All the Tools I have in my Garage: