High mileage maintenance

[and12buc]

Here's the deal. I have a 89 honda accord. not original owner. Has 273,xxx miles+. I have been maintaining it to the best of my capabilities and with tried the least costly way(not really). ive done two tune ups in the past 12,000 miles and used high grade components. added new minor things : radiator hoses, fuel filter, spark plug wires, o2 sensors, anchor bushing, blah blah blah. Kept up with oil changes every 3,000 miles. Oil i use: 3qts of Castro gtx High Mileage formula 5-30w, with an "engine restorer & lubricant" additive by Automotive Products. Also use engine flush each oil change.

Now my question is: is there a better way i could be maintaining my car along with better use of Additives. My main concern is the oil i use. Ive been reading about other high mileage oils and i want to know which is best for me.

Some say synthetic you cant go wrong with. but i also heard that its more likely it'll leak if i switch to full synthetic since my engine has very high mileage.
so looking on to the many high mileage formulas out there. i cant figure which is the best: Valvoline Maxlife, Citgo Ultralife, O'Reily's, AMSOIL synthetic. I cant decide!

And does the grade matter 5-30, 10-30, 10-40w etc. I was recommended 5-30w but would it be better if i change to a diff grade?

Also, about cleaning. I've read about Auto-rx. what are your takes. I doubt seafoaming will do good to my car since it might clean too much.

Let me know anything you can think of to better maintain. Thanks!