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Engine Oil. The Facts.

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Engine Oil. The Facts.

Postby Lazza » Sat Feb 22, 2014 7:00 pm

Some of you will know of John G (aka Skinny) from Nutz and Pistonheads. He is an oil expert that worked for BMW/Sauber F1 team so he knows his stuff. I asked him to do us quick write-ups, giving us the facts we really need to know about oil. He sent me the following details. Enjoy...

Oil viscosity ratings
All oils are thick when they are cold, and thin down with increasing temperature. Engine oils for the last 50 years however contain viscosity modifiers which don't have any effect when cold, but stop the oil thinning down so much when it gets hot. These are multigrade oils, and work effectively both at cold and hot conditions.

The two numbers on an oil grade are not related to each other - they are just an arbitrary scale for two completely different measurements.
The W (cold) rating relates to how cold an oil can go before it fails a cold crank and cold pump test.
The second number (hot rating) relates to the viscosity of the oil at 100C.

Basically, the lower the W number, the faster it will circulate round your engine. I'm not saying there's anything wrong with a 10W oil. But there's nothing wrong with a 0W either.

On the hot side, the lower the number, the thinner it is. Two 40 oils will have a viscosity within the same range at operating temp (100C), between 12.5 and 16.3 cSt. For example, two fully synthetic oils ; Mobil 1 0W-40 and Motul 300V 10W-40.

Mobil 1 viscosity at 100C is 14 cSt, and viscosity at 40C is 78.3 cSt
Motul 300V viscosity at 100C is 14 cSt, and viscosity at 40C is 89.5 cSt

Two things, both oils get much thicker when they cool down. They are also the same when they are hot. The W rating has no impact whatsoever on the viscosity of the oil when it's up to operating temperature. So all those internet experts claiming the 0W-40 you're putting in your car is like water... it's the same viscosity when up to normal temp as the 10W-40 they're probably advising you use.
And even when it's cold, the 0W is not too thin - the 10W is thicker than it needs to be. Now, you aren't going to do any damage by using a 10W-40 compared to a 0W-40, it is after all the correct spec (although bear in mind that 0W motor oils weren't commercially available 20 years ago). But just because the handbook doesn't say 0W-40, doesn't mean it's the wrong oil.

One last thing on grade - HTHS (high temperature high shear). This is how the oil behaves in bearings and measures how much it shears down (thins out). Many 5W-30 oils are A1/B1 spec. This means fuel economy oils - you get fuel economy through thin oil, i.e. it shears down a lot, gives less resistance but also less protection. You don't want this if your engine isn't designed for it. So make sure you get an A3/B3 spec oil.

Choosing the right grade.
Anything from 0W-40 to 10W-50 is fine for an MX5 and anything in between. Personally I wouldn't go for a 30 grade as it's a bit thin at the high temps. Also, I'm happy to run a 10W-60 in my car as it gets driven hard and the turbo puts a lot of extra heat into the system - but wouldn't recommend it for normal cars.

Tappet noise.
If you hear tappet noise, your tappets don't have enough oil in them. One reason can be they have gummed up due to infrequent oil changes / crappy oil. Another reason is the wrong grade. If they tap for 20 seconds when you start the car, your oil is too thick and isn't getting up to the top of the engine quick enough to fill them up. If they tap after a long hard session, your oil grade is either too thin, or it's thinned down due to the thickening additive getting chopped up (i.e. it's too old) and it's not holding the right pressure.

Synthetic vs non synthetic.
MX5 engines are old tech. They were designed in the era of mineral oil. So they are fine to run on this. But synthetic is better. Mineral oil is a cut out of crude oil and contains loads of crap, as well as having a wide boiling range. This means that as you heat the oil, the thin stuff cooks off and leaves heavy stuff behind. It's also more prone to oxidising when hot and leaving deposits and crap round your engine.

Synthetics are made from purer compounds so don't have contaminant crap. They are also a highly refined product so all the hydrocarbons are the same, you don't get light and heavy oil in the same can. They are also much better at resisting high temperatures. It's this purity that allows you to make 5W and 0W oils with synthetic base stocks. Mineral oils wax up and go too thick.

Part synthetics are a mix of both -but you only need a small amount of synthetic to call it a semi-synthetic. It's not half and half. But it does answer the question, can I mix oils. Yes you can.

All engine oils contain detergents and dispersants. This means that gunk gets removed from your engine when you do an oil change. So modern engines don't need engine flush - unless they have been run on poor oil which hasn't been changed for a while. You'll always end up with a bit of residual oil after a change. I'd rather have a bit of old oil than engine flush left in my engine.
They also contain anti-wear additives, and friction modifiers. All of these additives interact with surfaces and they form a carefully balanced cocktail. They have been specifically designed to pass all of the industry tests, and some manufacturer tests. So yeah, you can put some STP moly stuff in which reduces friction, but you'll make your wear and detergency worse. And DuPont tried to get Slick50 to stop buying their own Teflon (PTFE) as it was of no benefit to engines and just clogged up oil filters. Make of that what you will :)

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Re: Engine Oil. The Facts.

Postby Badknee » Sat Feb 22, 2014 7:27 pm

hellfire, I understood that, thank you very much. :obscene-drinkingcheers:
Paul & Angela.
'93 1.8 in Super Silver
Fully blinged up (thanks M-m)
Lowered into the weeds hung on Gaz Gold Pro's.

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