I’m kind of surprised after reading through all this that you let your oil filter go for 3 oil changes and 22,500 miles. Some people around here think the oil filter is more important than the oil itself (I personally do not).Yeah, that's not true. At all.
I don't personally know who "Imp4" is above but listen to him, too.
It IS true that if you run excessively long OCI's w/dino oil on them, you can sludge the oil as the rear bank does run hot, and oil pools in the head around cylinder 1 on the low side near the highest heat area. This was back in the day when people literally said, "It's a Toyota, I don't ever change the oil." I know that sounds crazy but I knew people who did that and that's where most of the myth arose. I have been to cocktail parties in Cambridge, MA where people boasted about not changing the oil in their Toyotas after 30k miles. It was a badge of honor.
I have a 2002 ES300 with 325k on it. Same car. I have run 7,500mile OCI's on it for the last 200k miles, using Mobil 1 10w30 HM. Oil consumption is 16 ozs every 7,500k and it was that at 100k and it is the exact same at 325k. I have also added some Lubegard Biotech to the oil for the last 100k miles, the ester base in that + additives really work.
If you want to take care of the engine, run an oil appropriate to your climate. That's likely 10w-30 unless you live in Alaska. NJ needs nothing weaker than 10w-30. If you want to do some feel-good stuff, run some Mobil 0w-40 like Trav and others said with 10 to 15 ozs of Lubegard Biotech for 5k. Then switch to the 10w-30 HM and additive and go 7.5k and you're fine. I run the Fram Ultra for 3x OCI's. 22.5k miles. Filter changes on these engines are annoying and messy so that's a gift from Fram; take it. I have cut-open past Ultra's I have run 15k and 22k and they were solid - have posted about this in years past w/pics. So run the ultra and feel good to run it to 20k miles or such.
This oil issue is really a red-herring. What you need to be thinking about with that car is to flush the fragile PS system - it runs hot and eats high pressure hoses and pumps. You also need to think about if the belt was done - likely not. The timing belt tensioners on these can fail, a thorough timing belt job is WAY more important on a 1999 Avalon with 100k than a Jesuitical debate about oil choice. Another thing to keep in mind is the 99 avalon was built in Kentucky and not Japan. While that's my home state and I say that with pride, you can't equate Japan-built Lexotas with US built ones; there is a long-term difference IME. Maintain this car and it will last forever, but don't neglect it.
I've worked on and serviced a lot of these cars, been a moderator at ToyotaNation in the Camry/Solara/Avalon sub-forums (a nightmare I don't want to repeat, btw), and also avid at clubLexus. I say that not to boast but to establish that I have not only my personal knowledge about them but also years of forum activity/moderation where I surveyed user/owner experiences and developed a deeper understanding of the issues and how they arose. I strongly suggest you not over-fret about the oil issue and look at the other long-term maintenance issues.
I do not at all intend to sound like I am talking down to you. That you did some research and came and asked means you care and that's great. Just trying to spread the experience I've paid for and learned, as I have benefited my whole life from others doing the same.
But good read. I frequent clublexus and Toyota nation, as well. In fact, the only reason why I’ve ever heard of issues with the 3.0 is from clublexus...some guy tried doing 20,000 mile oil change intervals, while swearing about the greatness of Amsoil...and blew his engine.
But good advice from you, seems reasonable (although the filter thing does blow my mind a little bit). I’ll have to check out that Lubeguard stuff. Usually not a fan of additives to oil but it does sound interesting.
And I agree 100%...the bad reputation this engine gets is overblown, there are 100’s of thousands of these things still driving around with 300,000 plus miles on them, with little to no engine problems.