Is the T3-15 the predecessor to the T5C. If it is, terramite corp told me to use 10w30 because I live in Indiana. I'm always calling terra corp for special parts information. 800-428-3772.
Another piece of advice is to clean the screen on the sump if it has one. Terra corp can tell you is it does.
Sometimes they will give me a part number to get tie rod ends and a seal for tranny. Terra corp is extremely helpful.
hope this helps.
geo
