You can use 10W40, with all the slop that is probably in the engine.
Here's what I would suggest though. Drain the oil, put the new filter on and put 4-5 qts of transmission fluid in it. Run it for about 5 minutes and drain it. This is an old motor cleaning trick.
Then put a new filter and fresh oil in it.
I would also start putting money away to do a rebuild or find another motor.