I did not use a tee at all on my install. i just dropped the sending unit right out beside the factory sending unit. All you need is some wire(12-14 gauge) and some terminal ends. You also have to tap into the 12v coming from the ignition so the gauge comes on and goes off as you turn the switch on. But i did not use a tee or any thing. take the plug out of the assembly that the oil filter is attached to and the adapter sent with the gauge will screw right in and then the sending unit will screw into that. make sure you use a sealant on the threads or else you will have a oil leak. Hope this helps. It took me about an hour and an half to install and that includes finding the 12v power source and everything.
Just in case you want to know, i was able to take the panel off the dash underneath the steering wheel and tap into the 12v wire coming from the key switch for the power. use a test light and find it. i think that it is a blue wire but i am not positive. There should be a bundle of wires going into the steering column. It is a 12v wire in there somewhere that only has power when the switch is on. That is the one you want.
Hope this helps and good luck on the install.