Rep Power: 0

just bought a 99 civic 4 door auto with 165k on it for my dd. i just put gas in it had about 3/4 of a tank and put 5.8 gallons in it. did my mpg and showed 21 very disappointing since i bought the car for gas mileage. Two factors i know of rear tire were low around 24 25 psi both of them, and the oil change sticker on my windshield says last time it was changed they used 10-30.Seems a lil thick to me. Main question What type and weight is everyone running?

 
Rep Power: 286

10w-30 is to thick, 5w-30 is what most people choose.

It probably needs a good tune up like most older cars; change the spark plugs, air filter, clean the throttle body, correct the tire pressure and see what you get.

 
Rep Power: 189

10w30 is just fine. Your car is spec'd for either 5w30 or 10w30. I use 10w30, and the Chevron Supreme brand because it has Molybdenum and other goodies in it. Most think it's 1 of the best traditional oils out there, can't go wrong by choosing it. Costco has the 12-quart packs for ~$21 sometimes, the normal price is $29.99 or so.

Ever since I got my car at 162K miles it has had a bit of piston slap, and with 5w30 the engine clattered at certain RPM speeds when cold. 10w30 cleared it up completely. MPG is up to 35+ on the highway. Tire pressure is 31 on all tires

 
Rep Power: 353

the first number is the cold start weight. if u live in a place that you start ur engien from 40-80, then you should stick with 5w.

the first number represents the viscosity at 32F (0C).

and ur tires should be around 30-35 psi

 
Rep Power: 189

Well I'm looking at the six gen service manual, and it's in black and white so hard to tell, but it looks like 5w30 oil is good from the ambient air temperatures of -20F (or less) to 100F (or more), and 10w30 oil is good from (not lower than) ~18F to 100F (or greater).

 
Rep Power: 813

stick with 5w30 for best mileage (esp if you live in places where it gets below freezing in winter). but they may have added 10w30 because of oil leaks. in the summer 10w30 is just fine. once the car warms up, the oils are the same.

 
Rep Power: 0

I live in east tx so worst temps it's going to see is maybe 20s. And I guess the problem was the tires but I'm still going to do the tune up

 
Rep Power: 0

I get 33 mpg highway/city with my 99 auto. I always use 5w30.

 
Rep Power: 0

Thanks for the input guys just filled up and got 38 mpg. Guess the Tires made a pretty big difference