I think I've been "in there" to inspect the oil pump innards on my slant (or small-block?). If so, I probably used O-rings from my nitrile kits ("Buna-N", black). I have an older large red kit which is getting hard to find these days, plus a few kits from Harbor Freight, one with metric-sized nitrile. For O-rings which touch fuel, always use Viton (brown or tan). Harbor Freight sells a kit. For AC use HBNR which are usually green but can be blue, red, or even black. HBNR is superior to nitrile for most fluids, but google to be sure.