I recently (1 month ago) changed the transmission fluid in my '08 G37s 6mt.

Here's the question- I filled it up until it overflowed while jacked from the front. I didn't leave the fill plug out and take it off the jack's to let more drain out while it was level. Should I do it now, or am I ok. I'm having no problems with it- just no way to check it obviously, so not sure if overfilling is a big problem or not. Advice?

 

You only jacked up the front when you did the transmission fluid change? It's important to have the car level when you drain and fill to ensure that you didn't put too much.

 

Google "aerated oil"

The Perils of Aerated Oil - Let Your Machine Burp

In a nutshell, too much is just as bad as too little. Both can lead to inadequate lubrication/overheating. Invest in a second set of jack stands or drive on ramps so you can get the car level when doing such things as trans/diff service.

 

Understood. The question I now have is: should I open it up and let it drain level until it stops?

 

The fill hole, that is.

 

When I did mine few weeks ago, I used drive on ramps on all 4 corners. I do have jack stands too, but was too lazy to measure the front/rear height differences. With drive on ramps, the car is similarly leveled as though it was on the ground, only 12" higher, and no, I didn't drive onto all 4 ramps at the same time. Each half (front/rear) of the car was jacked up first with a floor jack and ramps placed under front/rear wheels.

To answer your question, reread the importance of proper oil levels again.

I realize you're probably new to wrenching, but it is [critically] important to follow some procedure guidelines. Even in BB's DIY, he discusses that while only the front of the car was jacked up, proper fluid level was attained before sealing off the plug. Skipping over some steps will eventually be costly

Edit: To add to the above; In my earlier repair days, I failed to properly secure the the front end of a motorcycle while doing a fork oil service. As I undid one of the fork caps, it violently shot up. Had I been standing directly over it, it would of taken an eye out. The gouge it left is still present in the roof of the garage. Seconds later, the bike feel over. Needless to say, among other damage, the gas tank now had a huge dent in it.