The Motor Oil Bible

What does my oil actually do?

An engine oil's job is primarily to stop all the metal surfaces in your engine from grinding together and tearing themselves apart (and that's the last thing we'd want!). But it also has to dissipate the heat generated from this friction too. It also transfers heat away from the combustion cycle. Another function is that a good engine oil must be able to hold in suspension the nasty by-products of fuel combustion, such as silica (silicon oxide) and acids, whilst also cleaning the engine of such nasties. And it must do all of these things under tremendous heat and pressure without succumbing to fatigue.

 

Mineral or synthetic?

Mineral oils are based on oil that comes from dear old Mother Earth which has been refined. Synthetic oils are entirely concocted by chemists wearing white lab coats in oil company laboratories. For more info, see the section on synthetics further down the page. The only other type is semi-synthetic, sometimes called premium, which is a blend of the two. It is safe to mix the different types, but it's wiser to switch completely to a new type rather than mixing.

 

A couple of words of warning:

 

If you've been driving around with mineral oil in your engine for years, don't switch to synthetic oil without preparation. Synthetic oils have been known to dislodge the baked-on deposits from mineral oils and leave them floating around your engine.  It's wise to use a flushing oil first.

If you do decide to change, only go up the scale. If you've been running around on synthetic, don't change down to a mineral-based oil - your engine might not be able to cope with the degradation in lubrication. Consequently, if you've been using mineral oil, try a semi or a fully synthetic oil. By degradation, we’re referring to the wear tolerances that an engine develops based on the oil that it's using. Thicker mineral oils mean thicker layers of oil coating the moving parts (by microns though). Switching to a thinner synthetic oil can cause piston rings to leak and in some very rare cases, piston slap or crank vibration.

Gaskets and seals! With the makeup of synthetic oils being different from mineral oils, mineral-oil-soaked gaskets and seals have been known to leak when exposed to synthetic oils. Perhaps not that common an occurrence, but worth bearing in mind nevertheless.

 

 

Synthetics

Despite their name, most synthetic derived motor oils are actually derived from mineral oils - they are mostly Polyalphaolifins and these come from the purest part of the mineral oil refraction process, the gas. PAO oils will mix with normal mineral oil which means Joe public can add synthetic to his mineral, or mineral to his synthetic without his car engine seizing up. The most stable bases are polyol-ester (not polyester). What we meant by 'stable' is 'less likely to react adversely with other compounds.' Synthetic oil bases tend not to contain reactive carbon atoms for this reason. Reactive carbon has a tendency to combine with oxygen creating an acid. As you can imagine, in an oil, this would be A Bad Thing. So think of synthetic oils as custom-built oils. They're designed to do the job efficiently but without any of the excess baggage that can accompany mineral based oils.

Pure synthetics

Pure synthetic oils (polyalkyleneglycol) are the types used almost exclusively within the industrial sector in polyglycol gearbox oils for heavily loaded gearboxes. These are typically concocted by intelligent blokes in white lab coats. These chaps break apart the molecules that make up a variety of substances, like vegetable and animal oils, and then recombine the individual atoms that make up those molecules to build new, synthetic molecules. This process allows the chemists to actually "fine tune" the molecules as they build them. Clever stuff. But Polyglycols don't mix with normal mineral oils.

A quick guide to the different grades of oil.

 

 


Fully Synthetic

Characteristics

0W-30
0W-40
5W-40

Fuel economy savings
Enhances engine performance and power
Ensures engine is protected from wear and deposit build-up
Ensures good cold starting and quick circulation in freezing temperatures
Gets to moving parts of the engine quickly

Semi-synthetic

Characteristics

5W-30
10W-40
15W-40

Better protection
Good protection within the first 10 minutes after starting out
Roughly three times better at reducing engine wear
Increased oil change intervals - don't need to change it quite so often

Mineral

Characteristics

10W-40
15W-40

Basic protection for a variety of engines
Oil needs to be changed more often