Possibly a grain of truth in that for (older) petrol engines, but pay attention to the required ACEA spec, this is the European oil spec. (API is American, JASO is Japanese). That used to be A for petrol, B for passenger diesel, E for commercial diesel, and oils would be rated ACEA A2 B2 etc. The A/B ratings have now been largely superseded by a C rating for Catalytic converter compatibility. This in part refers to levels of the anti wear additive Zinc dialkyl dithiophosphate (ZDDP) in the oil, it used to be present in high levels but the phosphorous damages Cat converters and levels have been progressively reduced. The levels of Sulphated Ash, Phosphorous and Sulphur (SAPS) in oil is critical to most modern engines which will require an ACEA C1 C2 C3 or C4 oil, each of which is particular about levels of SAPS, and ensure the base stock and the additive package is suitable for the emissions control and fuel economy requirements of that particular engine.

As I said above I have a bit of an issue with the terms ‘Semi-Synthetic’ and ‘Synthetic’. To clarify, that is not because there is no difference in them, but because I feel the terms are often used misleadingly. The vast majority of oils marketed as Synthetic are crude oil in origin. This is not a problem but most people are surprised by this, and the technical definition of this type of synthetic (fully hydrocracked crude oil) is at odds with most peoples intuitive definition – never been in the ground… Now I’m not an expert on engine oils, I’m more into cutting oils, but to my knowledge Semi-Synthetic, Synthetic Fortified, Synthetic Blend, Part synthetic are all marketing terms which mean next to nothing. Oils marketed as such could contain very different amounts of base stocks in very different proportions. Take two hypothetical base formulations, ie before the additive is blended in;

Oil A, base stock; 70% group 1, 30% group 3. Costs say, 65p per litre to make

Oil B, base stock; 85% group 3, 15% group 1. Costs say, 92.5p per litre to make

Both could carry identical labels claiming they are both ‘Semi-Synthetic’ or ‘Synthetic blend’ or ‘Part Synthetic’ but the difference in quality will be significant, and the public would never know.