Looks like a grapefruit, but tastes like an orange
My dad recently bought a house that was built in the 1920's. There is a large citrus tree on the property. The tree is 20 feet tall at least. Right now it is covered in dozens of large yellow fruits that look like a typical yellow grapefruit. However, the skin is fairly thin and despite the yellow color of the flesh, the taste is very similar to that of an orange. In fact, one would mistake the fruit for an orange if tasting blind. It is very sweet and aromatic, not at all sour or bitter. I am guessing the tree is at least 35 years old or older, so it's not any new variety.
Any ideas on what exactly it might be?