I just completed installing an ATK remanufactured engine in my 95 YJ 4.0. I just change my break-in oil after 250 miles. I've been pretty excited that I didn't have any oil leaks. I changed the oil with 5W30 synthetic oil (still cold here in Utah). To my disappointment I have a small oil leak. It looks like its coming from my rear seal (oil pan looks OK) My question is.... could the viscosity of synthetic oil actually be causing my oil leak?