Those holes are all blind on the heads I have looked at here, so oil shouldn't be coming out through the threads. It is most likely the threads are stripped, or nearly stripped, and the bolt won't tighten up. (The head being different isn't really an issue, it is the thread diameter and pitch, and grip length of the fastener that is important, and a lot of that size bolt tend to have the phillips as well as a hex)
Take the cover off, clean the surfaces, check the threads (I always run a bottoming tap to clean them up, then squirt some carb cleaner and blow it out with air. Repair the threads if that hole is stripped.
If you don't have a gasket, you cam make one with a phone book cover. Use a thin film of grey RTV on both sides, or forego the gaskety and use RTV (a thin film, hand tighten the screws, with a day or so for it to cure then snug to torque.
About 2 gallons is right.