In layman explanation, the difference between general Grade 3 Fully Syn vs the others is FS is using Hydrocracked mineral component. Hydrocracked oil is refined to a stage that its almost comparable to real FS lube albeit minus several true FS inherent property such as natural higher viscosity index and better contaminant containment. However hydrocracked still have way better viscosity index and other properties than semi and mineral. Actually in general thats semi and mineral has more similarity. If you service at every 5K, it is arguably FS, Semi and Mineral generally perform almost the same. In real life tho, a good FS generally provide better anti-vibration, contaminant containment, smoother acceleration, longer lasting viscosity indexer, etc. In terms of engine protection, in our hot and humid weather, actually all good lubes be it FS, Semi and Mineral protect almost the same. In cold country, FS DO MAKE a big difference during startup protection (critical protection) due to its cold weather protection property (going up to -30 degree protection against lube feezing).

Thats one reason I mention even Mach 5 would not damage engine coz its still a good mineral (not fake or recycled), just performance suffers a bit due to extra engine noise, vibration, etc. If change every 5K, I don't see any reason mineral lube cannot be used. My City has used mineral up to 60-70K mileage and when open up engine, its still sparkling clean and all components looks new and THAT is using the original Honda and Idemitsu MINERAL lube. However as engine gets older, it tends to vibrate more, noiser, etc. FS with its better properties fixed that BUT engine protection still performed the same. The key word here is changing lube and filter EVERY 5K without fail. That is actually MORE important than using whatever lube, FS, mineral...in the capacity of engine protection. Performance is another story tho as mentioned in 1st paragraph.

Concept of FS is thicker or thinner than normal mineral and semi is not accurate tho...FS ability of getting thinner to suit high performance engine temperature is what mislead most ppl that FS is actually thinner. Hydrocracked and true FS do lubricate better and smoother and its viscosity changes from very high temperature range to extremely low temperature is the biggest difference here. Becoz FS has better natural viscosity index, it means when addictives that affect viscosity are worn out, it still are able to perform viscosity changes albeit at a much reduced range better than standard mineral and semi.

As for Semi vs Mineral, its still out there with a lot of arguing that it actually does not makes much difference between the 2. Its is given tho that mineral lube have slightly more contaminant than the rest. So semi is slightly cleaner and FS refined lube is the best on this aspect. However at 5K service interval this not as contaminant thingy is irrelevant. Usually the % of synthetic components of a semi is not given, we really does not know how much synthetic is there, maybe 10% or so. FS generally have higher syn components abt 30% but its the hydrocracked based oil that makes the biggest different, soley my opinion only coz we also don't know how much % of syn is there. Lube company keeps MUM...very secretive on that.

So generally I stick either with a very good proven mineral or go straight for FS. Semi IMO, is overated. I maintained semi currently becoz Proton uses it for my free service and parts up to 20K mileage. Probably will retain semi usage due to continuation but will change to either idemitsu or havoline semi next time. FS is too expensive for my 3 months engine lube change.

Oh....true FS like the Amsoil do have the best natural properties but most likely only effective for high performance engine where its properties truly shine. For ours, normal Grade 3+ FS might perform better.

ps. New Havoline with deposit shield just released, saw in Paul Tan site....ha....most prob will try its semi for my 25K change...next week going for 20K last foc service and parts....cialat have to pay over RM150 for extra gear oil changes since it is the only part not included in my FOC thingy.