JC, someone asked me the other day about the Castrol BOT130M oil and when I found the service schedule I was shocked to see 200,000km for the g/box OCI 
 
It does also say 20,000km/12 months replacement for the transmission fluid (and I'm assuming they mean the manual gearbox) under severe service, and I'd classify towing under that.
I've listed the service schedule below.
If you can't buy in bulk form Castrol try a Ford dealer, pretty sure it's the same 75W-90 GL4 fluid used in some Transit's, Fiesta's, etc.
[edit] Noticed you mentioned BOT382, does that supersede BOT130 ?
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