I too, am a big fan of Mobil 1, but with the age of your car and it's milage, I would want to know what type of oil your car has run on the past 11 years and if the previous owners had it changed regularly. I just recently purchased a '96 LS with 153,000 one owner miles. The owner took it in for every service, but only used regular oil. And the engine runs like a swiss watch. So I decided to continue with changing it regularly and use a high quality non-synthetic oil. There is also the possibility of synthetic oils causing leaks and seepage in older seals and gaskets. Sometimes it happens and sometimes it doesn't. That's another reason I decided to stick with regular oil. Synthetic oil still needs to be changed at regular intervals, and one thing nice about Mobil 1 is that you can purchase it at almost any big box chain store anywhere in the nation, whereas Amsoil can be difficult to find in some locations.
As for extended warranties, you really have to do your homework. I've only purchased an extended warranty once, on a new '03 Corolla S, and I got it through the dealer. It only extended the warranty period for 2 years. The warranty is now over, and I've never had a single problem with the car. So was the extra $800 bucks worth it? In this case, no. Like you said, there are alot of companies out there, all trying to get your money. I guess it all depends on the coverage that is offered. You've probably already checked the internet to see what's available. If you haven't already done so, check with the BBB to see if they have any info on the companies you are interested in. With a vehicle that has the age and milage that your car has, I would think it may be difficult to find decent coverage with a reputable company, because they know that the older the vehicle is, the greater the odds are that something will fail.
It's just a game of odds. What are the odds that your Lexus will have problems in the near future. Would you be better off just putting your money in the bank so it's there in case you need a repair, and if your car doesn't need repair, that money is still there, or paying out a load of cash to a warranty company today, only to never use it because your car doesn't break, or the company goes out of business.