2004 Corolla Auto 220k kms
Was planning to replace valve cover gasket
but the two rubber hoses do not come loose.
I was afraid using anything like WD40 to get them loose as they are rubber.
Should I use anything else?

I noticed no oil leaks around the gasket, but this probably has never been replaced.

Should I leave it as it is or still need to replace as the item is old?

second question is sometimes I feel there is slight lag/ miss when going up in speed.
one friend of mine, mechanic said I need to use injector cleaner to clean them, and this may go away.

His advice was let the petrol level go down (to let light on) add the cleaner, run the car until petrol almost empty.
then refill the fuel tank

Is this OK to do? injector cleaner bottle says add onto full tank