what grade oil synthetic or conventional in lexus es300 2002 with 132,000 miles?

Conventional is fine, synthetic does offer less friction but costs more.

I would go with 10/30. conventional. If you are using synthetic, it's best to stay with synthetic, if you switch, it can cause your oil seals to leak.

Use what the manufacturer recommends. It is in the owner's manual. No need to change.

It also would depend on the climate your in. If it's a southern state where it get's real hot, you can use a little thicker oil grade. The higher the number the thicker the oil. If your in a cold climate, then you need a lower viscosity oil so the car will start when frozen.