I had a leak on a 3i once, couldn't find where from so cleaned the block and used a flushing treatent that is not as thick as engine oil, ran the car on the drive and found the leak, oil temp sender, £2.50 and all sorted
Had a 1.6l rover engine, started making a squeeling noise from the top end just now and then. turned it over one day and it was rock solid, took the head off to find a vavle buried in a piston
Figure the camb must have been starved of oil and stuck, then up came the piston to meet mr valve
Clean the block down, fill it up with oil and run it, then see if you can isolate the problem.