From an engineering perspective, o-ring selection for this application is tough. You have heat, vibration, and petroleum products to deal with. Commonly available materials in this size and their resistance to heat/vibration/petroleum:
BunaN - fair/good/excellent (cheapest)
Silicone - excellent/poor/excellent
Viton - excellent/poor/excellent (most expensive)
As you can see its a compromise. The OEM silicone (and Viton) o-rings wear flat from pipe vibration and leak eventually. The cheaper and eaisly available in bulk BunaN (common black rubber) o-rings take a permanent set from heat and do the same. What I found to be the best solution is to use the BunaN parts, but use hi-temp RTV on them and the pipe/flange for assembly instead of grease, after cleaning everything up good with contact cleaner. This has worked very well for me, no leaks for months last season and none this year. Your results may vary depending on the alignment of your pipe/flange (from pipe bending) that puts greater side loads on the o-rings.
