That my friend is entirely a personal preference decision you need to make.
High mileage is like..formulated to help engines with over 75,000 miles on them, the base oil is the same, but the additive package is a little different.
Synthetic is really slick, works great in most applications, helps dissolve years of sludge in an engine, this can sometimes, on neglected engines, cause oil leaks as the sludge dissolves and oil is allowed to seep...but this is more or less a rarity, but it does happen sometimes.
Conventional, well...the engine has lasted 181K miles on it without a problem..right? Its cheaper to buy, and the engine is familiar with it.
Viscosity, depends on the climate you are in..if you are in a cold climate, a thinner oil is suggested to ease cold starting and to allow faster pump up times. A thicker oil is preferred in hot climates, and on worn engines...
The oil debate is a discussion that rages on in all forums, small engine, automobile, off road, racing, 2 stroke..wankle engine, turbo charged, whatever...people have an opinion on what oil to use and why to use it. Sadly, few people ever agree, and there is mostly hear-say and and opinions running rampant.
In my mustangs, Ford says to use 5W20, I personally use a 5W20 Synthetic to help protect the engine at 6000rpms+.
In my 5.2 ZJ, I use conventional 10W40, it has 226,000 miles on it...I prefer napa brand (valvoline) as its cheaper than valvoline in the valvoline bottle..
I change my oils between 3000 and 5000 miles, and I use Napa GOLD filters (wix premium).