breaking in gears

just got my yukon's yesterday. what oil do you all recomend for breakin in gears? the directions say to change it out again after the fist 500 miles. just wondering brand names and weight, or what works best. should i use synthetic?

i used reg gear oil for the break in forget what weight but that shouldn't be a big deal because your not going to be working the gears hard anyway. some say to drive 30-40 miles and then let it cool because a fresh set of gears can make some heat. after the break in use your high grade diff fluid with the weight that is right for your driving.

I've never seen a particular need for synthetic in normal applications. Supposedly they give you better mileage, but the difference is going to be too small to measure. If it's big enough to measure, your axle would be running red hot. Claims are that the synthetic does not mix with water, such as you might pick up in a water crossing, but most recommend changing the oil after big water. I will be a lot more likely to change cheap fluid than expensive synthetic.

As for the breakin procedure, like the man said, don't let it overheat. Several times I have installed gears and headed out to Moab. I just check them every 20-30 miles for the first hundred and then just drive them. I change out the fluid when I get back. More important than fluid type is getting the gears set up correctly the first time.