From Auto Trader user reviews:
Beware significant engine problems on 05-06 GS300 models.
RoadWarrior32 | 20th April '14
I bought my GS300 SE-L to replace a very reliable BMW 525i and given it was a Lexus, it just did not occur to me to conduct my usual extensive research as I just assumed reliability would be bulletproof. Well it isn't!!
The 300 has a DI V6, which Lexus launched with the GS in 2005 and it's the DI which causes carbon deposits to build up in the engine, which effectively strangles the engine, causing excessive oil consumption, low oil pressure and sometimes complete engine failure. O5-06 engines also suffer with defective piston rings, which effectively worsens the oil consumption to the extent that owners are often driving around with low oil levels in between services, which in turn can cause premature failure of the engine over time.
I found out the hard way when my 06 GS300 with Full service history and around 60k miles started to make a rattling noise, followed by a loss of oil pressure. Subsequent investigation found the engine filled with a sludge like substance, despite scheduled services with the correct synthetic oil.
You are looking at a bill for over four thousand pounds to rebuild the engine and this will not stop the carbon deposits returning as they a result of the DI. I believe Lexus redesigned the engine in 07 to address both issues but I'm done with Lexus, so you will have to do your own research.
Just search LEXUS GS300 Oil Consumption, Carbon Build Up or Piston Rings for more detailed info.