When Castrol mailed me their technical data I found it strange that they had a 20w50 listed there, since I had never seen or heard of it. Especially since Castrol has always promoted their 5w50 as being the perfect oil. But if it's so perfect, why the need for 20w50? I also found it strange that they had 10w40, as this is a very uncommon synthetic viscosity. Only Amsoil and Redline seem to carry it, but no others that I'm aware of.