All semi synthetic oils are mineral based as are all of the cheaper synthetics and many of the better ones actually have a proportion that is mineral based. There is a process called hydrocracking that refines and modifies mineral oil and after a court case a few years ago, the hydrocracked oils were able to be legally sold as synthetic. In semi-synthetics, the 'synthetic' part is always hydrocracked oil. Cheaper 'synthetic' oils are always made of hydrocracked oil to save on costs, although combining hydrocracked oil with a true PAO or ester synthetic, you can make a very stable top quality oil.
Engine flushes are not a good idea. Harmless deposits build up in an engine over time and removing them is often far more hassle than it's worth. Flushes will remove some of them, but other bits will just be loosened. You then put the fresh oil in the engine and drive it for a bit, the loose deposits can break free and you end up with hard bits in the oil. Not good. A better way to do it is to drop out the old oil, put in some really basic mineral oil, run it for 15 minutes, remove the mineral oil and put in the fresh oil and filter.
thanks mate
what really basic mineral oil would you advise to get form your site.. something cheap to do the trick.
ta