The information here has been supplied by the Register’s Technical Adviser, Alex Eacock, who is a senior family member of EMC Motorsport.
The 968 engine can trace it lineage back to the 70s and is a fabulous piece of engineering, even now. The mistake people make with these engines is thinking that the latest, newest, thinnest oils are better than older types of oil so they must be better for an old engine, which isn't strictly the case.
New engines have to meet strict guidelines on emissions and as such, very tight tolerances are applied to help with sealing the combustion side of the engine and preventing oil consumption where possible and by reducing friction.
Thinner oils reduce friction as they are moved out of the way easier and present less drag, to provide the lubricity needed on the bearings, they have high particle shear strength so effectively, the surfaces run on a thin molecular layer that stops them rubbing. The high shear strength of modern oils is useful in any engine to help prevent wear, however modern engines have tight bearing gaps which this thinner oil can fit into and works fine.
In a 968, which has fairly large tolerances on the bearing surfaces and also very large bearing areas, if you put thin oil into the large gap, it doesn't stay there. As an engine generates oil pressure from the crankshaft bearings, if the oil is falling out of the gaps, rather than staying where it is supposed to, you get a lower oil pressure, especially at tick-over.
Thinner oil is also easier to wipe off, as it has less body to it and wiping oil off moving engine parts is not good. So, with this in mind when selecting your oil, you need to be aware of its viscosity and try to get an idea of its additive package.
The minimum viscosity we recommend for a 968 is 10w40 but 10w50, 10w60, 15w40 and 15w50 are all fine. You can also use a 20w50 in an emergency but this tends to be a classic grade oil with less detergents and cleaners in and can cause oil galleries to get gungy with prolonged use and also tends to have less friction modifiers than a newer grade.
Motorsport oils are usually full of good anti-friction additives and stabilisers and come in the viscosities that we need for 968's but with the additional price tag and the caveat that the engine will need to be run up to operating temperature regularly, as they can absorb more moisture.
As a rough guide, we tend to use Millers, Ravenol, Petronas and Valvoline oils, as we have found these to be the best for the engines. Millers 10w60 CFS Nano is the best oil money can buy for these engines, however it is very expensive and difficult to get hold of. Ravenol and Petronas do a 10w60 that is very good and Valvoline VR1 10w60 is fine and all the latter three are readily available.