Hi Garry,
I have used belts designed for driving tape decks or record players on the engines I've made so far. These are flat with a thin, scale like, section and come in varying widths and diameters.
They do not appear to be affected by oil and do not relax their tension over time though I always take the belt off the pulleys when stored.
Not perfect to type but do have a look 'more in keeping' than 'o'ring material tried previously or the spring type.
Plenty on eBay to choose from and cost is quite reasonable.
Hope that helps
Ramon (Tug)
Edited By Ramon Wilson on 10/12/2020 12:07:26
Edited By Ramon Wilson on 10/12/2020 12:08:34