2 mile commute here. I bike when I can.
There are various problems with short trips. I'm not an expert, but some that I can think of:
-Burning fuel creates condensation=water. Ever seen water dripping out of a car's tailpipe? That happens when the exhaust isn't hot enough to keep it from condensing. The inside of the engine gets hot quickly, but it might be a while until the oil is hot enough to get all the moisture out of the system.
-The engine and oil never really get up to temperature. Oil works best at operating temperature, and it's too thick before it warms up. Running 0w30 or 0w20 would helps since the oil is less viscous at startup, but still has the necessary viscosity at operating temperature. I've read that you should avoid full throttle before the engine is fully warmed up, and even then the oil might not be fully warm for 30 minutes of driving. Hot oil is watery compared to how it is when you pour it out of the bottle, but the engine is designed with that watery viscosity in mind.
-The transmission probably never warms up. It's full of oil which presumably works better warm. Almost all vehicles have a transmission cooler built into the radiator which actually acts as a transmission heater as the engine warms up. Vehicles that are designed for heavy towing may have another transmission cooler in front of the radiator.
-Same with the diffs and transfer case, but these are less complicated, so they are probably fine unless it's extremely cold out. Synthetic fluid will help.
-The battery probably doesn't charge enough to make up for starting and running all the lights and stuff. If your battery voltage starts getting low, put it on a trickle charger
-The engine burns extra gas when it's cold to warm up the cats faster. If you have a V8 the secondary air system runs for the first 30 seconds.
-Rubber parts are less flexible when they are cold, so more likely to crack. I doubt this is much of an issue though.
For frequent short trips I recommend installing a block heater even if you live in a warm climate. If you have a V6 it's super easy to do, and the hardest part is finding the hole and routing the wire. If you have a V8, it's a bit of a pain since you have to drain the coolant from the block, and access is difficult. I plan to do it when I do the timing belt and water pump, probably this summer. Here's the best thread I can find anywhere on a V8 block heater install:
http://www.toyota-4runner.org/4th-gen-t4rs/179593-04-v8-block-heater-install-question.html
Other options include stick-on or magnetic block, oil pan, or transmission heaters. Starting with a warm engine should save lots of wear on the engine, and it should save some fuel, too. On cold days you don't have to idle as long to warm it up/clear the windows.
With some creative wiring, you could wire up a block heater, a trickle charger, and any other heaters you want to one plug. For the winter it would be awesome to wire the seat heater and rear defroster to the circuit, too! You would just have to find a way to tap into the circuit to apply 12V power and isolate that from the car side so you aren't driving 12V into something that's supposed to be off.
Check if the owner's manual has a "severe duty" service schedule and follow that if you want. With synthetic oil it might not matter since you can go a lot longer than the normal interval anyway.
