API CC, any higher will not be a good idea, i had to glaze bust and re ring a bunch of hand crank listers which had been run on Castrol RX Super 15w40 which was something like API CE. I removed an oil filler cap on a running engine which was a bit of a pig to start(they all were) and it shot the filler cap the length of the workshop followed by a blast of oil which left a black sprayed line on the underside of the roof about 10ft above. All the engines were badly glazed in the bores. The glaze bust and re-ring cured the starting while using the manufacturers recomended spec of API CC.