Hmmm, well here are teh PDS of both the oils:
5W30: http://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/AECC776DF8C45D2280257CE700137286/$File/CASTROL%20EDGE_5W-30%20A3B4_3374731_2014_01.pdf
5W40: http://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/C6E03139D0B614D080257C2B001472C8/$File/CASTROL_EDGE_5W-40%20SN_3774718_2013_11.pdf
The 40 has higher KV and HTHS but that's to be expected. Both claim to be FULL synthetics but neither state what base stock they use. Would be interested if anyone could find out.
I also found this:
http://www.synthetic-motor-oil-chang.../#.U-cYGGMivu4
Pretty interesting read, although a bit of advertising from Amsoil themselves of course. Interesting to find the favorites (RedLine, M1EP and Castrol Edge Titanium) either doing really well or really badly. That annual cost thing is a load of crap though.
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