I literally just experienced the same problem with my 99RX300 AWD. Serviced by dealers rigorously, oil changed within recommended intervals. Saw oil usage and asked dealer to check compression. 2 cylinders were low (~90lbs). Hoped it was valve stems/seats. But, ...had 2 cylinders that had mirrored over and reduced compression. Apparently the rings fused to the pistons and cross hatch on cyl walls was gone. Used premium oils, including Mobile 1 for the last 50k. Engine seemed to have plenty of power but was idling rough and appeared to be using oil...which was unusual. During the diagnosis we found the Idle Air pump was going bad and it was probably reason for rough idle. Ended up replacing short block (cheaper than fixing) and rebuilding head assys. All new seals/gaskets, valve covers, etc. I was shocked since I had 148k miles...mostly highway....and had pampered this thing since day 1 and have been the only driver. Lexus was not interested in investigating since no signs of sludge. They said it just wore out. A very poor explanation for an engine so rigorously maintained......especially a Toyota engine. Good news was that the dealership cut me a huge deal on doing the work (I think they were embarrased quite honestly) and I did the engine, tranny fluid service, and new rotors/pads all around for $8k flat. Car is now under warranty again for 12mo/12k miles. A huge disappointment for me that this happened. Now wonder if Lexus is really all that since I've never had any other car with this issue no matter how many miles it had...even an old K-Car with a gazillion miles on it.
I would suggest having the basics checked and serviced as normal and get use to it burning a little oil. I would have been better off leaving mine alone and waiting for it to fail completely. Once we had it opened up that far to diagnose it, it wasn't a lot more to replace the short block as opposed to putting it back together as-is. If you want to know, a compression test may be very revealing. Even if you're low on one or more cyls but have power, etc. you still may want to ride it out, watch your oil closely, and see if you fall out of love with the car before it fails. It may be a lot cheaper. Fortunately, I don't have the strut issues and my car still drives like it's new with almost 150k miles. Even after having a new RX350 for a month at no cost while they did the work.....I still like my '99 better. Doesn't feel like a cream puff.
The biggest disappointment in this whole thing was Lexus had no interest in any kind of detailed follow-up to understand what caused this problem. Dying of old age at 148k miles on a Toyota engine of that size is ridiculous.....although Lexus wouldn't agree. My mechanic was the one that compelled me to follow-up with them since the engine was so clean and was clearly maintained well. He was miffed as to why it failed. I know every company has to manage its costs and warranty claims and am not shocked that they didn't want to put any effort into it......but then again, they're supposed to be different. Just another car company in this case. However, the dealer gets a gold medal for the way they helped, gave me a huge discount, and a loaner the whole time.
Good Luck!
Wow! Interesting. I'm almost scared to even do a compression check. I'll just run this sucker into the ground - and when I do I'm picking up another SUV - Probably American.