I don't know, I'm totally new to this kind of stuff, usually I hand them the key and pay the nice lady at the counter for periodic maintenance. But if using a better oil will help in the cold and last longer between fills, I'm sure going to figure out what it is so I can specify that it be used.

 

Yes, they should have 0w-20, the newer cars use 0w-20, even camry's use 0w-20

 

Is piston slap bad? How do I stop it or fix it?

 

I spoke today (January 2015) with Lexus of Windsor, 9379 Tecumseh Rd E, Windsor, Ontario, Canada Phone:+1 519-979-190; the parts department advised that while 5W30 is in the original spec sheet for use with the RX300, a Lexus publication shows that 5W20 synthetic or 0W20 synthetic are also approved for the RX300. Apparently 5W20 is the most commonly utilized in their service department.

The foregoing Lexus recommendation is consistent with a very educational article about motor oils, which dispels many myths, and which can be found here.