I bought a 99 Avalon new. For the first 125,000 miles I had the oil and filter changed every 5,000 miles at a local, independently owned lube shop. I liked the place because you drive your car over the pit, get out, and watch (and talk with) the owner while he services your car. I used Castrol 5W30 dyno and his filter (Wix). I then changed to Castrol 10W30 HM dyno, which I ran until 175,000 miles. I then began changing my own oil regularly, and went to M1 10W30 HM (with a M1 or PureOne filter), which I still use.
At 210,000 miles I replaced the timing belt & water pump. I had developed a leak from both valve covers, so I pulled them to replace gaskets and plugs. The valve train had varnish, but no sludge. In my case, the 5,000 mile OCI's with dyno worked fine. If I had known of the sludge problems in 99, I would have used synthetic from the beginning. Having seen the valve train, I am going to up the OCI's to 7,000 miles using M1 10W30 HM and either a M1 or PureOne filter. If I catch the Mobil 1 oil/filter on sale at Advanced, I buy it. Otherwise I buy a jug at WM and the PurOne filter at Advanced. Either way, it's about $30 and cheap insurance.