Synthetic ATF is definitely a better product. It lubricates better, cleans better and works better when cold. It has actually fixed transmission shift problems for me, probably issues that had their root in gummed up valve body assemblies.

But don't vary the maintenance interval. Yes, it is expensive to change with Mobil 1 ATF.

On the other hand, how expensive and time consuming is a whole transmission rebuild?

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles