On Voltair (4-108) we have always used Gates “Extra Service” belts type 6568 EXL (AVX13 x 1088La) costing about £15 each. However, although the length is just right (easy to change without removing anything) we do chew through about 1-2 per season and cover the engine compartment in black dust, so I am not convinced these are optimal.
One problem is that different makers measure the belt length differently (it’s not always the length around the outside of the belt which is mentioned). We noticed this in Ireland (where else?) when we couldn’t get our usual type.
John Kingsley