so sorry for late reply, its oil + sludge, not copious amount but enough to float and stick to the rubber tube in the expansion bottle, theres also mayonnaise under the rad cap, bottle drained, washed, refilled, then oil sludge comes back in about a wk time to accumulate, since i noticed this problem i drained engine oil in a big pan, no sign at all of oil/coolant mix, just pure old engine oil. nothing on dip stick showing coolant.
the kit my mate used was one of those u put blue colour solution into the tube and place over the rad top filler hole to check for gas leak, which just stayed blue (not turning yellow), my truck hasnt overheated or lost power at all. actually it goes better after the Terraclean treatment. she leaks oil to mark her territory everywhere she goes so i top her up small amount twice a month, not really losing much coolant but level does go down max about an inch from the top mark on the expansion bottle, i only need to top it once or twice a month, just small amount needed. shes a 96 has done just over 180k with first gen single point LPG.