My 96 Honda Civic EX is rated for 5W-30 all year, or 10W-30 during the summer. The engine has 168k miles on it, and the guy who usually changed my oil says that when an engine gets over 100k miles on it, its time to "bump up the oil" and so he's been putting 10W-30 in it all the time, instead of 5W-30. However, this guy also overfilled my Honda, when I said "It says 3.8 quarts max", he put in 4 quarts stating "Eh it'll hold 4." (Above the top mark of the dipstick 3/4ths of an inch when engine is cool) In other words, I question his knowledge. So, even on a high mileage car, could I use 5W-30? I dont push the engine hard very often, and everything I've researched says that lower viscosity means it will flow better on startup, and that most engine wear occurs at that time. So would 5W-30 be better for my engine than 10W-30, or has engine wear after 168k miles warranted a constant useage of 10W-30 to protect it? Also, conventional or synthetic?