The alternative hypothesis is that Toyota is going out of their way to recommend an oil that's going to blow up engines, increase warranty claims, and cost them a lot of money. Which would you believe?

 

Since Lexus hasn't really made the new oil recommendation public (via mailings, etc.), I'm sure you could continue running 5W-30 without worries.

 

I'm not terribly concerned. I switched to M1 5w30 at my first change, and I will most likely continue to run it even if Lexus makes the TSB more widely known.

M1 5w30 is very low on the 30 weight scale:

M1 5w30 has a 9.7cst at 100C. To qualify as a 30w oil, cst needs to be 9.3 - 12.5 @100C. So at 9.7, it's on the verge of a 20w already.

EDIT: actually, I take that back. I just checked the most recent data sheet from Mobil on 5w30, and the cst@100C is now 11.3. So I may go to M1 5w20 on the next change.