What oil should I use? Edit
This is one of those discussions that verges on a Religous War. All types and brands of oil have their violently passionate adherents.
Straight Mineral OilEdit
This is oil out of the ground, refined, but with essentially no additives. It does not suspend particles of combustion or grit, and it's only use is for initial break in after a rebuild, precisely because it does NOT decrease wear - your piston rings will seat faster, and you're going to throw this first charge of oil out after no more than 1000 miles in any event, no matter what oil it is.
Fixed Viscosity Detergent OilEdit
Same as above, but with a modern additive package, which will have anti-wear goodies, as well as detergents to keep the tiny nasties in suspension. This author prefers this type of oil.
Also known as single weight oil, so will only have a single number for its thickness (ie 30W for normal 3 season use in the UK).
Single weight oils have the advantage for air-cooled engines that they are more robust to high temperatures.
Multi-Viscosity Detergent OilEdit
A thin oil with additives that alter the "Viscosity Index", so that it doesn't get as thin, as fast, at high temperature, as it would without this package. Mainly an aid in cold starting. Remember this - 10W40 oil is 10W (thin) oil with a fancy suit on. It's anti-wear properties are WORSE than running the PROPER single-grade oil, say, 30W (for most 3-season conditions).
Also known as multi-weight oil, in simple terms, this oil acts like a thin oil when its cold (ie 10W) so is easier on the engine during startup and when cold, but then behaves like a thicker oil when its hot (ie 40W), this is why multi-weight oils are described with two numbers ie 10W40, or 10W30.
One criticism of using multi-weight oil for aircooled engines is that it may not cope as well with high oil temperatures which are often experienced by air-cooled engines, this can lead multi-weight oil to break down and lose its multi-weight characteristic, leaving effectively a thin oil.
Starting with very simple molecules to make bigger ones, created by science and not by dead dinosaurs, with better properties than mineral oils in a number of ways. MUCH more expensive, and the increased drain intervals that make them worthwhile in modern cars are USELESS in an aircooled Type I/Type II/Type III VW due to the lack of an oil filter, and the crude crankcase ventilation system.
A compromise - some of the increased properties of synthetics, at a lower cost by cutting them with mineral oils. Again, not worth it (over fixed viscosity detergent oils) for our engines due to the need for short oil drain intervals.
What oils shoud I use again?Edit
Some oils are reputedly better than others at the typically relatively high temperature a VWs oil sees compared to a liquid cooled engine. Castrol has had an excellent street rep for years. You will not go far wrong chosing it in the single weight (Castrol HD) viscosity appropriate to your climate. Morris 30 is a common choice in the UK, and is also a single weight oil.