Which oil should I use

[Ericwest]

Hello everyone! I just bought a used express and could really use some advice. I know it has 5/30 viscosity oil. But do I use conventional or synthetic? What did it have when sold new? It’s a 2003 express 3500 with vortex 6.0. Any help would be greatly appreciated

[MountainManJoe]

I've split off this question into a new topic since it's completely unrelated to your other one.

Picking the right oil really couldn't be any easier. Open up your owners manual to the maintenance section and find the oil specification. Then all you have to do is buy oil that meets or exceeds that specification.
If your manual was lost, download one from GM: https://experience.gm.com/support/vehic ... als-guides

Check the labels on the bottle. Look for the official API certification marks.
Focus on specifications that are actually regulated.
Do not pay attention to unregulated marketing jingo like "synthetic", "high mileage", and so on. They're lies.
Brand does not matter. The most important thing is making sure the oil is full, and changing it every year or 5,000 miles whichever comes first.

unnamed.jpg (69.58 KiB) Viewed 186 times

attached: API engine oil guide

Motor Oil Guide 1020.pdf

(2.04 MiB) Downloaded 13 times

[Ericwest]

Yea. Was curious because some say using synthetic in a van that previously had conventional oil will cause oil leaks. I have the manual. Searched high and low for oil specs. I will download another. Thanks for your help

[MountainManJoe]

some say using synthetic in a van that previously had conventional oil will cause oil leaks.

Nonsense. So called "synthetic" oil will be just fine in your van. You don't need to worry.

Searched high and low for oil specs.

Should be under Service and Appearance Care > Checking Things Under the Hood > Engine Oil