Syncro 4&4 Discussion and Q&A last answered over 2 years ago.
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Daryl at AA Transaxles in the states goes with the Swepco now the MT90 has been shown to not do the job; it's what the porsche boys use
I'll stick with the semisynthetic and regular changes advice, I've also seen a few damaged by "too good" an oil so I'm not convinced by the fully synthetic stuff