Cheap regular gearoil is fine, GL4 grade. Get the thinest GL4 on the shelf, like 75w80 or something like that. 75w-90 is what the manual says. Not over 90w or 120w as it may not shift. But if the syncros are old, you can use Dextron as a thinner gear oil. If you have new syncros, don't use synthetic or anything that costs too much, or the syncros can't wear in and you won't be able to shift. It will always feel cold.
1.7 litres I think.