I've tried most of major brand syntetic oils for my TwinCam 20 namely Mobil 1, Castrol SLX, Shell, Penzoil, Syntium & also Toyota ori Fully syn.
Surprisingly, there only one that gives me real satisfaction in terms of slickness, low noise, long lasting, power, temperature etc.
Q8 Oils!
After using all these fullysyns some1 recomended me to Q8 so i switched to Q8 semi 10W-40. I can say that this Q8 semi feels better than other brands Fully syntetic...
Then I switch to Q8 Exel 5w-40 Fully Syn and im impressed with its performance especially above 5000rpm.. i guarantee u can feel d RPM needle reach the redline faster.
Giv it a try bro. no additives needed. Just the oil.