I was just wondering what the deal is with semi synthetic oils. While i can see that fully synthetic ones are excellent, i can't really justify spending at least 50 dollars for 5 litres of oil in my old car which is using just a little oil as well (worn valve stem seals), especially when i change it every 5000 km (okay- i could extend change intervals with synth). For about 25 for 5 litres i can get mobil semi synth in the correct weight for my car (15w-50), which seems like a good compromise. Is semi synth just a gimmick, or is it actually better than mineral stuff/junk? Any pref on brands (altough i think mobil is probably the best of the easily available oils where i am). Thanks for your help...