Some people are decidedly weird! I make a few favourable comments about Idemitsu's 0W20 and suddenly some nut-job out there in Blog-land is using my 'endorsement' as proof positive that this is the best oil ever invented. Well just for you, whoever you are, here are a few things to mull over...
If I'm right, the Idemitsu 0W20 (the version described in Volodymyr's tables) is formulated with PMA VII. This is relatively rare as the bulk of the world's engine oils are formulated with OCP VII and a small percentage (typically top-tier oils) made with HSD VIIs. The reason PMAs are so unpopular is because they are polymerically very inefficient. You might need two to three times as much PMA polymer in an oil to achieve a given viscometric balance (as in KV100 & CCS). This assumes the PMA VII is a high (50+) SSI VII. If you move to a lower SSI PMA, they become even more inefficient. Poor efficiency means far higher cost. Also very high polymer loadings generally mean far more piston deposits, especially in diesel engines.
However high SSI PMAs are interesting in that for a fixed KV100 & CCS, they will yield a lower KV40 than either OCP or HSD. This manifests itself as an apparently high Viscosity Index (VI) but this is somewhat misleading as the VI, as defined over the wider viscosity range (say KV100 to CCS temperature), is the same for all VIIs. Low KV40 is however useful if you are looking to achieve fuel economy improvements, especially from cold start when FE is generally at its worst.
Japanese oils will often be put together with PMA VII and an abnormally high treat rate of Moly (1000 ppm is not unusual). Moly is a very effective friction modifier so the two things together tend to give some of the best FE oils, especially if slotted into the thinnest oils like 0W20's. However high treats of Moly have their downsides. First all Moly additives are crucifyingly expensive. Second, Moly additives often contain nitrogen (eg Moly Dithiocarbamates) and can, at high treat rates be very aggressive to Viton seals. In the days when I could blend my own oils for my own car, I shied away from using high treats of Moly even though the oil cost me nothing.
So, to sum up, if like me, you are obsessed with fuel economy and the need to stop global warming, drive everywhere at 55 mph max and never go above 2,200 rpm, you own a small, very cheap car which arrives new containing a 'free' sump-full of super expensive Idemitsu 0W20 then I heartily recommend you enjoy the FE benefits it gives you. However, if you have to pay for this uber-expensive oil with your own pennies, drive a diesel, drive fast and hot, are looking for something beyond simple SN/GF-5 and are looking to keep you car for the long term, then maybe this oil ISN'T for you.
