John deere 425 mower won't start

Here's the link and I'm sure you will get the solution of your problem.
John Deere 425 Lawn And Garden Tractor Mower Service Repair Manual

some use straight 30 some 5-30 others 10-30 jd has their own oil as well the following chart showes the type

Here's the link and I'm sure you will get the solution of your problem.
John Deere 425 Lawn And Garden Tractor Mower Service Repair Manual

Check fuel line. It could be stopped up. Also check inline filter or in tank filter for trash.

Check for a dirty fuel filter if the unit has one. Look in the tank with a flashlight to see if there is dirt in the tank.If there is dirt the tank then there is dirt in the carburetor. It will need cleaned

Sounds like the carb is varnished up and needs to be rebuilt. They make rebuild kits depending on the carb number and you can rebuild it.

I had this same problem with my John Deere LA140. I suspected that it was some sort of fuel delivery problem. Long story short, it turned out to be the gas cap was not venting properly, creating a vacuum in the fuel tank. The temporary fix was to loosen the cap when it starts to die. This relieves the vacuum and allows the gas to flow again. Installing a new cap on the tank fixed the problem.

Check your fuel filter and see if it's clogged. You might be able to blow it out with an air hose if it is.

When it dies loosen the fuel cap and see if it will start.If it starts its because the tiny vent hole in the fuel cap is plugged.

Don't run it until you've completely drained and flushed and refilled the fuel system, tank, lines, pump...
Don't run it! Diesel fuel is more of an oil whereas gas is more volitile... Avoid starting or running to prevent any damage to your engine!