I am poor so it'll be Valvoline sold as "Parts Plus" (the name of the local auto parts store), but what weight should I be using for the truck. It has about 100K miles on it.

 

I'd go with whatever the manual says, but others might suggest using a higher weight oil because of your rig's age, but stick with what the manual says if you don't have any engine problems such as burning oil, loss of compression, etc. Hope it helps

 

I don't have a manual! This is why I ask. It has a slow oil leak but doesn't seem to burn any. I have so far used 10w-40, but I need to know what the manual calls for since I don't have it.

 

If it's cold there right now go 10/30. Come summertime, bump it up to 15/40.

 

OK, Winters here range from 20F as the extreme rare low to about 55F, and averageing about 40F. Summers average about 68F with lows of 50 and highs of 90. Is 10W40 good all season oil for a moderate climeate like mine?

 

Yes. I change the viscosities of my oil a couple times a year just to accomodate the different seasons. Your owners manual should have a little chart in the maintence section.