First, I don't consider 80k miles high mileage. But as for leaks from switching over to AMSOIL, generally if there was a problem with a "false seal" before, it will show up using AMSOIL.
Let me explain some things that I'm sure you will understand once you think about it. When you use a petroleum oil for a long period of time, a varnish-like deposit forms on the walls and other surfaces of the inside of your engine. If you have ever seen the inside of an engine, I'm sure you have noticed it. This varnish material is not sludge, but is a brownish coating. It doesn't much matter what brand of oil you have been using, it still forms, although it might vary in color.
This coating will tend to cover the seals in the engine. The most critical are the front and rear main seals and the valve guide seals. This coating keeps the oil from contacting the seals, which will cause the seals to dry out and shrink. A seal will swell as much as 10%. This is an important factor in keeping an engine tight and free of leaks.
In the 1970's when synthetics were new, the seals were not all made of the same materials and some synthetics treated them differently. If you where ever into an old Chevy small block from the early 1960's, they used a piece of rope as a rear seal. Now seals are all pretty much the same stuff, regardless of the vehicle manufacturer, and the various oil manufacturers know what they are dealing with and have overcome any adverse or inconsistent seal swelling.
Anyway, anytime you change oil brands, and it doesn't have to be from conventional petroleum oil to a synthetic oil, it can be from one conventional petroleum oil to another brand name petroleum oil, the fact that each oil has a different chemical make up will often remove this varnish. Synthetics are, of course, different and will usually lift this coating off. AMSOIL, since it is formulated to last longer for extended drains, has a high amount of detergent/dispersant additives and will clean the inside of an engine. When this coating is removed, it re-exposes the seal. Often, the build-up of varnish was providing a seal, or "false-seal", and by removing the coating, the seal will start to leak oil. On really high mileage engines, like over 150,000 miles for gas, and 600,000 miles for diesels, the seal may have cracked. This seal will obviously keep leaking. But in most cases, the seal will re-swell after 500-1,000 miles and stop leaking.
You can't really blame the oil because it is only doing what it is supposed to do -- clean the engine. I always recommend using the AMSOIL Engine Flush. Swap out the oil filter with a cheap filter (don't drain the oil). This puts the crankcase about a quart low. Pour in the flush and let the engine idle for 20-30 minutes. Don't run the vehicle on the road, as this flush thins out the oil.
After running the flush through, drain the oil (allow it to drain thoroughly) and swap out the cheap filter. Put in the AMSOIL oil. If the engine was fairly dirty when it was draining, I always recommend swapping out the filter at 1,000 miles and again at 2,000 miles. The reason is the AMSOIL oil will continue to clean the engine and the dirt and varnish being removed will be deposited into the filter. With all this dirt, the filter is likely to blank out prematurely, the by-pass valve in the filter will open and you will be circulating unfiltered oil. The unfiltered oil deposits dirt behind the rings, binding them up, and you get blow by. You may have heard someone say they changed oil brands and after 1,000-1,500 miles they started using oil. They changed the oil back and it stopped. All they really did was replace the blanked out filter. By changing the filter a couple times before putting on the AMSOIL Super Duty Oil Filter, you will be continuously cleaning the engine. The oil is still good, so there is not reason to change the oil. Just put in the make up oil necessary after changing the filter.
Now, as for the transmission and differential, because the seals are not subject to the heat and acids that are a byproduct of the combustion process in the engine, these seals have nowhere near the problem. In installing AMSOIL Synthetic Automatic Transmission Fluid into the automatic transmission, however, you need to change ALL of the ATF, not just what is in the pan. I change out the pan and filter, usually about 4 quarts. Replace the pan and refill. Then disconnect the line from the radiator that returns the fluid back to the tranny. It's best to test to make sure you got the right line. Bump the key to see if fluid is pumped out of the line or out of the tranny. If its the tranny, reconnect the line and disconnect the other one!
I put a clear plastic tube (available at the hardware store) and run it into a bucket. Mark it at 4-5 quarts, or what you think is still in the tranny. Have someone start pouring in fluid while you start the engine. As the tranny pumps new fluid into the torque converter, it pumps old fluid to the radiator and back into your bucket. When you hit your mark in the bucket, or see it change colors in the plastic tube, turn off the engine. Reconnect the line, and top off the fluid after running the engine for a while. You have pretty much replaced all the fluid and will see a big difference in the performance of your tranny. Check the fluid often for a few days in case there were any air bubbles.
The differential is not as simple. Just drain it when it's hot and try to let it drip-drip-drip as long as possible. Then put in the AMSOIL Synthetic Gear Lube of your choice.
Never use a flush in the tranny or differential.
