I dont want to sound like an arse, but you guys dont know what you are talking about!
Semi synthetics are not made up of a mix of synthetic and mineral, in most cases they are a mineral base thats been hydrocracked to perform like a synthetic. Base oils come in 4 different groups 1, 2, 3, and 4
Group 1 = Basic oils like chain lubes.
Group 2 = Most Oils like engine oil, gear oil, hydraulic oil, industrial oils
Group 3 = Highly Refined mineral oil (Semi Synthetics) High performance Lubricants, Engine oil, gear oils and other high performance lubes
Group 4 = Polyalpholefins or POA (Fully Synthetic) (Man Made) Extreemly high performance lubes for high temperatures or extended drain intervalls
Group 3 oils are sometimes listed as synthetic because the chemical structure of the base has been changed by man so therefore its man made and hence called synthetic.
I hope i haven't up set to many people but i had to explain how it works or im going to go nuts reading some of these posts.
Any Semi synthetic or fully synthetic is going to perform well above what you guys are going to put it through, so long as you change it at regular intervalls.