Gallons? Oil is usually measured in quarts - comes in quart containers. I assume you plan on doing an oil change. I would strongly advise you to take it to a lube center and have them do it this time for you. I'm certain if you ask to watch to learn they will show you everything you need to do for an oil change and will tell you exactly how much oil your truck/car requires for a complete change. The reason why I am sending you to a lube specialists is because you also need to see how much oil comes out, see what kind of an oil filter you need, see where that filter is and the tools they use to remove it, etc, etc etc. Lacking an owners manual is relatively easy to get around. Chilton's puts out "owners manuals" for every make and model and year of auto on the road. Newer models (going back to even the 1970's in many cases) one an find a book at any auto parts store. If you have an unusual or rare or older vehicle one can go to Chilton's them self: http://www.chilton.cengage.com/ If I had to be pressed I would say that about 4 quarts is what is needed. Mind I can not find a specific website on the duramax engine with specifications. However I usually use about 4-5 quarts on my truck during a change. I use the dip-stick method - I pour in a quart insert the dip stick, get a reading - if I need more, put in more...

Service manual says 10 Quarts (9.5 Litres) or 2.5 US Gallons With a oil filter change.(Sorry Athrael)

2 gallons

dont ever take your truck to a jiffie lube or place like that , they will use regular car oil on your diesel , it will ruin your engine desiel oil and car oil are diffrent . go to autozone buy a gm oil filter and 2.5 gallons of oil .diesel oil .. lift up the truck 4 inches with a jack and stand.. plug is a 16mm its metric . dont use sae or you will strip the plug.. oil is going to drain fast and a lot of oil very fast. dont use a small container i use a 55 galllon plastic drum cut in half maybe only a foot . it works great .. you can buy A BIGGER OIL CATCH PAN AT WALLMART. after oil is done gushing wait 5 minutes.. put plug back in . use quit a bit of force the diesel vibrates harder then a gas engine .. very snug .wipe away excess oil with clean rag..oil filter take off with big pliers or buy a wrench.. let it gusj too in the drain pan.. make sure the rubber gasket is off the engine part.. fill new filter with oil half way its a sideways filter .. oil rubber seal on top of filter.. tighten filter hand tight as tight as you can with bare hands, no reason to over tightn .. its a wives tail .. refill engine at top where there is a plastic cap ..top front of engine ..replace with exactly 2.5 gallons of oil ..replace cap. check dipstick to verify corrrect fluid.. start truck . keeping a eye on the oil presure gauge should come up to 30 psi to 40 psi in 45 seconds .. keep engine running check for obvious outside leaks... done..

2 gallons of Shell Rotella T multigrade and 2gts of Lucas Oil Treatment and you cant go wrong! Using Rotella T oil and Lucas Oil Treatment,I've put 1,000,000 mi. on a 60 series detroit diesel, before an inframe overhaul.