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MG MGB Technical - Diesel Semi-Synthetic Engine Oil

I recently bought a 1989 Land Rover 90 2.5TD which has 195K on the clock and which runs on 10x40 semi synthetic diesel oil very happily. It occured to me that given the higher zinc content of diesel oil and the purported advantages of semi-synthetic oils over conventional mineral oils, plus not having to stock two different types of oil, it may be worth running my 1973 MGB GT (with 90K on the clock) with this instead of the usual 20x50 mineral oil I have been using. I would be interested in any comments the forum members might have on this.
PJ Eades

I had been thinking of using a diesel oil myself, due to the zinc content, although not necessarily semi-synthetic.
Dave O'Neill2

Paul - anecdotal but make what you want of it; I had a similar thought about my 3.9. It had had some work and needed a higher detergent oil to clean it so I refilled with 10 - 40 semi as usual but chose the diesel variant (having no cats). She ran normally for a short while (say 1000 miles) and I began to to hear the sound of valves in trouble. It didn't occur to me that the Rover petrol engine didn't like the diesel oil and I assumed that the new set of lifters were at fault. I have replaced the lifters - many of which were damaged, and of course, the oil. She runs normally but at the back of my mind is the possibility that the detergent removed some of the protection for which the zinc was not able to compensate. On the question of viscosity; you might find that your MG would burn more 10 - 40 than 20 - 50. Even though the oil changes would be less frequent, I don't think you would benefit from a semi either.
Although you had a good lateral thought, I would stick with 20 - 50 plus a zinc additives and frequent oil changes unless you are racing in which case you might revert to Castrol R (castor oil) or whatever it's called now.
Roger W

I think the pumps are less happy with the thinner semi's too.
Allan Reeling

I used diesel oil for a long time and was more than happy with it (Fuch) but my supplier at the time told me that ZDDP was being phased out of diesel oils as well.
I now use "Penrite classic light". Penrite have a complete range of oils for classic cars from veteran through classic to modern. The classic light is a 20/60 with 1600 ppm of ZDDP well above the recommended 1200 min
and the highest I could find in any off the shelf oil. My engine has been supercharges for ten years yet when we pulled the engine early this year at 100,000 miles the crank was dead standard and even the cam was spot on. There are other good oils out there but Penrite have all the spec for there oils on the WWWeb for all to see.


I used Halfords Enhanced Diesel 15W40 precisely because of its higher ZDDP content, although it would be wise to stick to something that gave the petrol as well as the diesel API classifications, like the Halfords, and not use anything that indicated it was suitable for Diesel only.

But since then I've found Comma Sonic 20W50 which has an even higher concentration of ZDDP It's also likely that Diesel concentrations for 'current' engines will reduce over time, as petrol oils did.
Paul Hunt

Of course, BMC did produce diesel versions of the B-series engine, although they wouldn't have been particularly high-revving.
Dave O'Neill2

My V8 engine was worked on by Real Steel in London (American V8 engine specialists). They recommended Valvoline VR1 20-50 and I've used it ever since, changing it every 3000 miles. It is specifically formulated with high zinc and phosphorus for flat tappet engines and stays nice and clean in my 3.9 engine.
Mike Howlett

How can you tell the ZDDP content of an oil? is it in the API classification.
Can someone explain The figures.
Trevor Harvey

there seems to be some debate about the ZDDP levels required and that other elements in the oils can continue the level of protection

each individual oil can be made up different so its not about API standards - (you'll need to cut & paste this link)

car oil is a lot more complicated subject that it first seems and as with a lot of things it gets different groups with different beliefs

a tech sheet (or safety sheet) will usually give you information but like petrol (with is made up of hundreds of things) getting exact information seems to be commercially sensitive

if you want to find out about oils here's some links to get you started, add in more details from different suppliers/manufacturers - let me know what you can precisely pin down

(then there's the fun of what is a true synthetic oil)
Nigel Atkins

Paul H., Are you using the Comma oil in your V8 ?
Mike, I have used Real Steel in the past and respect their advice. The VR1 oil has a high zinc content but I wonder about the choice of viscosity and the low detergent statement [Given the Rover V8 tendency to sludge]??
Roger W

Roger - yes, and the roadster. However I've not found any tendency of either to sludge, after nearly 100k when I had the inlet manifold and valley gasket off the V8 it was a light golden brown inside, which equated to 'low mileage' according to RPI. Mind you I've always changed the oil at 3k intervals.

I did try Valvoline VR1 but I didn't like the containers which are the ones used by companies that sell cheap reclaimed oil, one was short on quantity, the claims for ZDDP don't always stack up, and I'm sure the engine got noisier after using it.

To get the ZDDP figures you have to research the manufacturers web sites, bearing in mind that oils with nominally the dame name and viscosity can have different levels of additive according to the market it is sold in. Most are below 1%, some a lot below, whereas Comma Sonic was 2% last time I looked. Comma Classic is less than 1%, and is probably what is in the Halfords Classic cans.

Castrol XL - a very 'old' oil recommended in some quarters for its high ZDDP content, actually has less than the various GTX formulations for example, which itself has had the quantities significantly reduced over the years.

The whole area is a minefield of confusing and conflicting claims and information.
Paul Hunt

It certainly is something of a minefield and thanks for all your comments.

I have decided on balance to go with the Comma Sonic 20/50 as I checked the ZDDP figure and they still quote 2%, which is significantly higher than their 'Classic' and Halfords 20/50, which according to what I have read elsewhere, is the same oil anyway.
PJ Eades

Castrol also markets an oil designated as Castrol 4T 20W50. It is specifically designed for air or water cooled off road engines running under extreme duress. It meets the stringent Japanese JASO requirements for street and off road motorcycle engines. It contains a very high level of ZDDP and a higher detergent package. I've been using it for five years in all of my vehicles and am quite pleased with its performance. RAY
rjm RAY

I'm currently evaluating Penrite Shelsley Light 20w/60 in my V8. It certainly improves the idle pressure!
Allan Reeling

This thread was discussed between 28/11/2013 and 03/12/2013

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