Yes... Don't use it. It will swell all the rubber seals in the engine, and in the end, when you finally get around to fixing the rear main seal, you will also be replacing the front engine seal and all the valve seals, because they will look more like sponge rubber than a rubber seal. In the old days, the quick fix was to put a few ounces of brake fluid into the engine oil. People did this with engines in cars that leaked oil that they were trading in. There is no such a thing as a mechanical fix in a can. It is just usually a quick fix that doesn't work in the long run. If this were a daily driver that needed to last another year, then I would say use the quick fix, and if it doesn't' work, the car is going to be gone in a year, so who cares. Since we plan on keeping our collector cars for a long time, then fix it correctly the first time. I know a number of people in this club that look for the cheap remedy only to have to fix it more than once. Most often when a rear main seal starts leaking, the engine is due to come out for a major overhaul anyway, because of age and use.