Kroil is probably the best penetrant commercially available. The small squeeze can is the cheapest and will last a long time. Some of the better hardware stores carry it, or you can order it from
Amazon or Eastwood. A friend of mine had a business that overhauled turbochargers and said it was about the only stuff that would work on parts which had seen very high temperatures. We also used it in the jet engine business on fasteners that had seen over 1200F. Zyglo or FPI fluid, used for Florescent Penetrant Inspection on nickle and aluminum alloys also works well because it's designed to penetrate tight cracks, but I suspect it's a lot more expensive and harder to get than Kroil.