Understanding engine oil grades

[Julian B]

This question is not unique to Haflingers, but is still relevant to them. I am in the (fortunate?) position of running three older vehicles, and would ideally like to use just one grade of oil in all three. Allowing for likely temperatures that I will be using them in (approx -5c <> 30c), the Handbooks say

• 1956 Citroen Traction Avant SAE 20 ("All temperatures")
  1962 Haflinger SAE 30 (0c <> 30c) or SAE 10w30 (< 0c)
  1964 Mercedes 230SL SAE 10w40 or SAE 10w50 (-20c <> 30c). Not sure why two are given. (For 0c <> 10c, SAE 20w40 or SAE 20w50 is suggested, and in reality I never use the car in sub zero temps)

I have been using Castrol GTX 10w40 in the Haflinger and Citroen, and a 20w50 in the Mercedes.

Seeing as "20" and "30" both sit between the range "20w50" would it be possible, realistic, or "right" to use just the one multigrade oil, or am I missing a key aspect of understanding oil specifications?

Or do I owe it to myself to just buy three grades and use the specified one in each machine?


[EDITED TO ADD: I see that Steve's recommended Valvolene VR1 20w50 is available in the UK; it costs ~GBP 30 inc VAT for 5 litres, which isn't cheap ]

[HaffyHunter]

Hi Julian,
I can't say I know much about Citroen engine design but I should think that 10W30 should be fine to use in it as 20W is exactly mid range of the multigrade.
The Hafi functions well with 10W30 and it is preferable if you operate in cooler climate.
The Merc 230SL is a bit of an issue as I know the engine tolerances and oil galleries were relatively large and depend on higher viscosity oils for proper lubrication.
So if I were you I'd go with two grades being 10W30 and 20W40 or 20W50 if the Merc is only used on warmer days.
Yes, VR1 is pricey but I prefer to pay more for oil with zinc than to buy motor parts. VR1 is available in many different grades. Alternatively there are new oil additives coming to the market to add zinc to standard oils although this may put the price on par or more than using VR1.
Cheers,
Steve

[heinkeljb]

Oil is a "can of worms"!

I agree, you should probably have two grades of oil available, the 10W40 and the 20W50. Whether you go for high zinc, modern low zinc won't really matter if you change the oil more frequently than the recommended mileage.

(Even experts don't agree on what is best or why you should / shouldn't have additives in it. The thing about oils is that they have a "working" temperature. You have to use an oil which is the correct grade when the engine / oil is at the working temperature. Now, I know most people won't go out with a thermometer to measure the temperature of their oil and the air temperature every day for a year to see what range the temperatures fall into.

So most manufactures of vehicle produce a chart which covers air temperatures from below freezing to hot. The multigrade oils like 0W30, 10W30, 10W40 etc are supposed to tell you the characteristics of the oil at any given temperature. So 10W40 oil should act like a 10W oil when it is hot (i.e. pours more like water than oil) and thickens to 40W when it is cold. (i.e. pours more like treacle when it is cold).

Most engines are designed to run at 100 ~ 120 Degrees Centigrade (water temperature), Oil temperature is likely to be a bit higher. The trick is to use an oil which has the right "thickness" when the engine is at "temperature", - this works beautifully for your straight oils like the SAE 20. The problem is that when the engine / air is cold, the oil might be too thick to be pumped round the engine properly thereby allowing engine wear to happen. That means you need a thin oil so it can be pumped round without requiring a heavy duty oil pump and large oil galleries, but then you need it to be thicker when the engine heats up!

Like I said, Oil is a can of worms)

John

[HaffyHunter]

I agree with John's comments except that replacing modern oil more frequently in older and flat tappet engines is as effective as using oil with zinc content. It is well documented among vintage vehicle, racing fraternities and oil manufacturers that this does not work to replace zinc.
Cheers,
Steve

[Julian B]

I found this article today, & it is written in laymans terms. Probably a bit UK (/EU?) focussed for Steve, but still interesting.
http://www.classiccars4sale.net/classic ... assic-oils

[HaffyHunter]

Hi Julian,
The article mentions using diesel motor oil in gasoline (petrol) engines and this has been practiced by many classic car owners in North America. Their preferred choice of diesel engine oil has been Shell Rotella T. However, Shell has recently advised that they have dramatically reduced the zinc content in Rotella T so I don't know if this is an industry wide trend or specific only to Shell.
The article also mentions that synthetics offer superior wear protection which is undoubtedly true but synthetics are not compatible with Haflinger engines due to seal designs.
Cheers,
Steve