Semi Synthetic is only suggested for the IS because it doesn't require any more so you would be "wasting" your money going fully synthetic.
Lexus/Toyota don't recommend either type, they just publish the specs the oil must meet. Those specs are in your handbook.
yea i saw the spec list in the handbook and remember 5W-30 semi-synthetic. I just wasn't sure if using fully synthetic would be detrimental to the engine in some way (but couldn't imagine why). I was alos told that with the s/c fitted to mine, fully synthetic oil would be wiser than semi - but again, not sure why??
I'm not sure the handbook states semi-synthetic but 5W-30 is certainly one of the recommended grades.
With a SC I would recommend using fully synthetic oil. Synthetic oil can tolerate greater pressures and temperatures over mineral and semi-synth oil and also offers better engine protection for the same viscosity.