What is the best type of oil to use in Chevy Avalanche with 80000 miles on it?

alot depends on the condition of your engine. Has the engine been maintained religiously with oil and filter changes? What kind of oil pressure are you running or do you just have a light?

You asked TYPE..........so does that mean synthetic or regular SAE oil? There are blends of the two also.

There is also a high mileage formulas made by several companies that I run after 75,000 miles. It is proported to have extra additives to keep valve stem seals soft.

If your engine is in good shape, I would stay with what the manufacturer suggests as far as weight. If the manufacturer calls for 5w-20w then I would stay with that viscosity. They make high mileage formulas in most all weight oils. You can stay with regular type oil and not use the high mileage fomula but I woudl add some LUCAS OIL STABILIZER. It's the only additive I use other than a product called RESTORE.

There are some additives I think or feel are snake oils.

One thing I use is a can of Restore when I change oil about every 25,000 miles. This product is made to fill cylinder wall scratches and improve ring sealing. I would add a can of that in place of about a quart when you change.

There are oils that claim to go 15,000 miles, but the big key to engine life is, how long does your oil filter last?

It is senseless to put in a 15,000 mile oil when your filter is clogged at 5,000 miles or you are going to change the oil at 3,000 or 4,000 or 5,000 miles

And a lot has to do with how you drive. Do you tow? drive in dusty conditions? short trips? stop and go? highway?

There's no one flat answer to your question, it takes the owners judgment using as much info on the condition of engine and how you drive and what your budget is.

I wouldn't go for synthetic but there are others out there with their opinions and you can get all sort of answers to your question.

If it were me and the engine is in good shape, I would stay with the viscosity that has been used or is recommended by the manufacturer. Also the temperatures where you live affect oil viscosity. People in Alaska have to use different weight oil then people in Arizona.

I would stay with what has been used in the past, add a can of Restore NEXT oil change, then at the next oil change I would either switch to high mileage or go with regular oil and throw in some Lucas Oil Additive.

Make sure you use a quality filter to keep your engine running a long time. When the oils detergents, and lubricity break down and you throw in a dirty filter, your engine's life is being shortened the longer you drive it like this.