I’m referring to engine oils here, the base stock is classed in 5 groups, the first 3 of which are derived from crude oil. Group 1 is ‘mineral’ oil, Groups 2 & 3 are mineral oils further hydrocracked to remove impurities. There is no real description of what “semi-synthetic” actually means, and can in reality be any blend of base stocks, though “fully-synthetic” would mean 100% group 3 base stock.

So, any seal compatibility with these should be similar as they are essentially the same oil, just cleaner, typically with less sulphur content. ‘Synthetic’ oils tend to be thinner than mineral oils (SAE 0w, 5w, 10w as opposed to SAE 15w, 20w etc) and might find faults in seals that thicker mineral oils don’t). What really does start to confuse things are when groups 4 and 5 start to be included. These are things like PAO synthetics and Esters. These are expensive and are not common in most passenger car oils, but they do find their way into Motorsport and very high performance oils. I’m not really able to offer much information on these without going very far from my comfort zone!