My car had an oil leak, and over time this made the rear of frame very, very dirty. There was a pretty substantial cake of oil all over the frame and engine surfaces. Since the block had a hole in it and had to be discarded, there was no point in clean it, so I concentrated on cleaning the frame.
I had previously removed the engine mounts; they were quite rusty, and while they still seemed strong I decided it would be best to replace them. They come off pretty easily, with two bolts holding each in place.
After knocking away all the loose rust, I was left with an incredibly dirty frame. My first step was to remove the thickest stuff using composite plastic gasket scrappers. These are nice since they don't damage the epoxy.
After scraping away the sludge, I was left with only somewhat dirty frame. I used Simple Green and shop rags to wipe away the rest of oil. This worked quite well, leaving behind clean epoxy. That was really all there as to it.