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Oil Specifications etc

I've always been 'religious' about changing oil at the specified times and using the correct oil etc.

True, I tend to buy from Halfords - provided they stock something which meets the specification (so far they haven only failed to do so once) but I've always been happy with their own brand oil.

I noticed today when changing the oil in Senior Management's Smart Car, using the recommended oil I happened to have 'in stock', all the spec numbers were identical to some Diesel Oil I'd bought for my previous MH (a Fiat Euro 4)- including the key Mercedes Benz one.

The oil I'd bought for the mh is clearly marked Diesel whereas the other oil isn't and, I assume, for petrol cars (which the Smart Car is).

This raised a question in my mind- is there any real difference between oil for diesel engines and petrol engines?

Needless to say, that led to another question. I use 5-30 oil as that is what is specified although 5-40 would be acceptable as the key number is, as I understand it, the 5. 10-30 wouldn't be suitable. So, if I had some 5-30 and some 5-40, could they be mixed without risk of damage to the engine?

No, I'm not trying to skimp/save money etc. I'm just curious. Penny pitching on oil changes is, in my mind, false economy. Even though we do under 5k miles per year in the Smart, the oil is charged etc year.

Reply to
Brian Reay
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There's a great deal to an oil specification other than its viscosity. Plenty of which is hard to find out.

For example, the old Rover V8 needs a suitable oil to prevent premature camshaft wear, being a pushrod engine with very high loads between cam and tappet. Modern engines are overhead cam, so less loading.

The best spec for this (generally accepted) is a high ZDDF (zinc) additive content. But you'll not find that quoted on the tin, with most. And that additive is not good with catalytic convertors. Nor does it seem to be mentioned in SAE etc specs.

Reply to
Dave Plowman (News)

Agreed- which is why I low for beyond just SAE.

But, presumably, Rover (or the parent company) specified it somehow and it is 'buried' in a spec number. True, given it is an old car, the spec may not be listed on current oils. If not, only oil specially made would be suitable which is hardly likely.

A parallel is the Merc Benz spec I noticed this morning (I can't recall the numbers now). I expect is covers several things.

'Low Ash' seems to be a new thing- my mh specifies an Oil which, it seems so far, is only available from Fiat. I expect there is something which is the same, or will be, once once the Euro 6 engine has been out longer.

Reply to
Brian Reay

low ash oil has been around for quite a while, often referred to as C4 oil iirc. the merc oil type (622.?) has to be programmed in at oil change, so it knows when to tell you to change it.

Reply to
MrCheerful

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