Why are there various engine oil grades ?

[DIYspanner]

As far as I'm aware there are the crankshaft shell bearings and the various designs of valve operating gear, basically either OHV or OHC,

so why do different engines need different grades ?

 

[Burnerman]

Machining tolerances are the most important changes of late, which affect viscosity grades, together with high flow oil pumps and more oil galleries to take the lube to where it is needed! Obviously additives are there for other functions such as cleaning, reducing soot production and so on.
John

 

[Dave54]

The oil cools the engine as well, particularly the bottom end. Modern oils tend to be lower viscosity. So presumably the faster the oil gets around the engine the better it cools it. Also as John says modern synthetic oils are "low ash" which stops particulate filters and the engine in general getting gummed up with soot.

 

[Nige F]

And modern oils suspend the dirt - Old oils if left long enough will let the dirt settle out . Something I found when I bought an old but hardly used rotavator . The oil at the sump filler was clear but yellowish. Drained it and the oil was black as Newgate's knocker. I reckon it had sat in a shed for 30 years unused.

 

[Burnerman]

Lets hope Peugeot read this with their 1.6 diesel ......not Peugeot's fault I admit, but it doesn't take any prisoners, that one!
John

 

[Avocet]

Turbo engines place really tough demands on oil. The increased use of turbos has done a lot to drive oil technology forward. Also, as John says, machining tolerances have improved immensely. This has allowed the use of thinner oils - which give a corresponding benefit in cold startup performance (they can get round the engine quicker) and fuel consumption (less internal drag to sap power). Oil technology itself has improved so some of the different grades now available in synthetics is simply "because they can"! Valve lift has increased a lot. When I was a kid, a "full race" cam might have had (say) half an inch of lift. Nowadays that's pretty common on road engines (and, of course, they have twice as many valves).

 

[Agile]

RIP my Peugeot turbo. Waiting to fit new cheap inner bit.

But I have always wondered when diesel oils have good particulate suspension capabilities then why are they not better to be used in a petrol engine?

Tony

 

[Avocet]

In a lot of cases, I think they are? I certainly know one engineer who goes out of his way to shop for diesel oil for his petrol car based on exacly that theory. Whether or not it's sound, I don't know. I think diesel engine oils have more detargent in them (which you probably wouldn't want in an oil if you could possibly help it!) and I think petrol engines work at higher temperatures. Also, petrol engines rev higher, so maybe petrol oils have more anti-foaming additive? I don't know though, I'm just guessing.