Subaru Impreza

Greetings friends, I have a request for your help. I have a 1996 Subaru Impreza 1.6. I recently had the engine rebuilt with all new parts, but the hydraulic lifters are making noise. A friend told me to use Mobil 20W-50 oil to prime the lifters, but it's currently using Mobil 10W-40. I'm not sure which oil is best or what you would recommend for a recently rebuilt engine like this. Thank you in advance.

Use a 15W40 oil because if you use a heavier oil it becomes more difficult for it to penetrate through the small holes of the tappets when the oil is cold.

Brother, viscosity varies a lot between brands. I'd tell you to drain them and manually fill them outside the car. If they drain when you start the engine again, you need to make sure the oil pressure is building up in the cylinder head (just because the oil pressure light goes off when you start the car doesn't mean it's lubricating the cylinder head properly). If the engine has been completely rebuilt, NEVER, EVER ADD ADDITIVES! Never!