My view would be yes. The concern to get GL-4 is to avoid corrosion of yellow metal components in the gearbox, but I suspect that for most people the concern is overstated. Synthetic oil is probably a definite advantage for the overdrive, which tends to get excessively worn, and also overheats easily, so the slipperier synthetic is an advantage.
For all the oils in the vehicle, it must be realised that virtually anything you can buy today is going to be a lot better than what was specified when the vehicle was new.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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