My two cents:
Anyone that has torn down an engine run on Conventional oil, knows it breaks down leaving a thick black carbon film (gunk) on internal engine parts. This carbon will clog PCV, oil filter and oil galley's and coat everything over time. Clogged PCV will increase internal pressure yielding a leaky engine. Conventional oil's also quickly loose their lubricating ability as they break down. The coating of internal parts will also reduce cooling efficiency, resulting heat will accelerates this "breaking down" of the oil. These are some of the reasons why Conventional (even blends) need changing every >3K miles IMHO. Additional the gunk builds on and damages seals.
Synthetic oils solve all these issues, and will actually clean out the gunk leftover from conventional oils. This includes cleaning off seals, which then "may" leak, not something we see much of with Toyota's. If any leaks develop, adding High Millage M1 or a seal modifier will usually take care of them.
How long to go between changes also has to do with time and operating condition. Where as EP M1 with M1 syn filter is guaranteed for 15K miles, I'm not comfortable going that long in city stop and go traffic or with frequent cold starts. It would take me one or two years to go 15K, which one years is about the max I like going between change. Whereas on long HWY runs, racking up the miles, I'm quit comfortable extending the miles between changes.
I also like using M1 synthetic ATF. Many say never flush (full 12qt), especially a high millage transmission. The concern is it will clean seals leading to leaks, and even more of a concern is the transmission fiction plates. Many transmission shops say the plates are being held together by gunk, clean and they'll fall apart. Where this may be true on a Honda, I've not had any issue with our rigs. But regardless I figure if the transmission is that fragile, I may as well find out now.
Note: Re-torqueing of valve cover bolts is necessary no matter which oil has been used. This will stop most valve cover and spark plug tube seal oil leaks.