x2 on Bigjoe's post. Most catalog stores are probably going to sell the complete gasket set for your T-150; not expensive but I'd go to your local parts store and see if they can get it for you, you may get it quicker and won't have to pay shipping.
Don't forget to change your fluid if it's been in there awhile or you're not sure how long it's been in there. If you fill it from the top while you have the cover off, make sure you don't overfill it (shouldn't come above your fill plug opening on the right (passenger) side of your transmission.
The good thing about removing the top cover is that you'll be able to pretty well visually inspect the inner workings.
"We and the world will always be at war. Retreat is impossible. Arm yourselves for the conflict." Leif Enger