The only thing not mentioned above is that the XL series is not formulated for use beyond 7500 miles and is not what this board would consider a true synthetic. The other Amsoil oils and the s2-3000 are synthetics. The XL series is labaled synthetic but a hydrocracked petro product. Produced for the Quick Lube market and those not wishing to play around with their warranties as the XL series is also API certified. The others meet or exceed but are not certified and do not carry the ole API doughnut symbol.