My car has a tranny with 130+k miles which replaced the original tranny that died and which had 110k. with that in mind what grade oil should I use, 5w10-30, 40, or what?

I live in Florida, I understand the weather may be something to consider?

 

For an automatic tranny, you should use Honda ATF-Z1 fluid. Not engine oil.

 

For the engine - the manual says 5w30, but you can use 10w30. From my research, it looks like 5w30 yields better mileage than 10w30, but since its thinner, can leak more in older engines.

The Tranny should have nothing to do with the engine oil.

 

What about 10w40? what is it used for?

 

If you have the 5-speed manual transmission, the owner's manual specifies engine oil with a now-obsolete API SF or SG rating. Depending upon ambient temperature, you can use the following oils:

Above 20°F - SAE 30
Above 0°F - SAE 20W-40
All temperatures - SAE 10W-30, 10W-40.

Lower viscosity oils have flow easier than higher viscosity oils so you should have slightly better fuel economy with a 10W-30 than you would with a 10W-40. Dino (conventional) oils use Viscosity Index Improvers (VIIs) which allow the 10W base stock to have the hot viscosity of a 30 or 40-weight oil (ie 10W-30 & 10W-40). Being polymer additives, VIIs shear down to a lower viscosity with use. You can also consider using synthetic oil as these have minimal or no VIIs to shear down. Either way, you're only looking at 2 US Quarts to fill your transmission.

I would also consider using a Heavy Duty Engine Oil (HDEO) as these contain higher levels of anti-wear additives than Starburst oils. Since you're in Florida, you could easily use 15W-40 as this viscosity is generally very economically priced (20W-40 is very difficult to find). You could also use 30-weight HDEOs (0W-30, 5W-30, 10W-30) in your engine as well.

Engine Oil Article