I bought 5 litres of Fuchs GL5 gear oil and changed it every 6,000 miles after having Scrimingers rebuild the FD at ~69,000 miles. Remember to keep an old gear oil bottle with tube, much easier to use and just refill it as required.
Halfords 75w90 gl5 semi synthetic. Changed every 3k along with the engine oil (filter every 6k). At £10 a bottle (which annoyingly doesn't quite do the box and FD), it's not worth trying to make it last - cheap insurance.
I also use the same Halfords oil. If I could easily get GL-5 EP90 (i.e. monograde) I'd use that, because that is what Getrag recommended for the 5-speed boxes, irrespective of ambient temperature.