[sick472]

I cannot help with the OEM size of thread, but I have always found the nylon threads to be the best for more heavy duty applications. The best of the best is called Bonded Nylon Upholstery Thread. I would take a sample of your old thread to a place like Hobby Lobby and just compare thickness by eye and you should be able to find a very close color match. It's not a seat, so it won't see severe duty use...you just want it to look original, last a long time, and be easy to re-apply. Error on the smaller/thinner side so that you can use a needle that passes through the existing holes easier.

Hope that helps!