My G35 has 135000 miles on it and I noticed that the transmission is starting delay/slip as I press the accelerator at slow speeds. I read that a complete flush could do more damage. A mechanic suggested putting more fluid in it just to see if the level was low. Any suggestions?

 

I suggest checking the fluid level first. Get the tranny up to temp (go for a 15 minute drive), park the vehicle with the engine still running, manually shift down through each gear and let it sit in each position for a second or two, then back up through each gear, put the vehicle in park with the engine STILL running.

Dipstick requires a 10mm socket to remove the retainer bolt, remove, wipe, replace, do this 7-8 times to get all the extra fluid out of the tube, then take 3 more readings wiping the dipstick each time. I believe the difference between the high and low hash marks is exactly one PINT which is 1/2 quart.

Just out of curiousity, have you been neglecting the maintenance on your transmission? The pan should be drained and filled every 30k miles. I question the reliability of your mechanic if he didn't check the fluid level and is suggesting to add more...

Here's a good read on automatic transmissions and changing fluid.

https://g35driver.com/forums/g35-sed...ml#post7123536

 

he did suggest checking the levels before adding fluid. He took the bolt off but there was no dipstick there.

 

Go down to your local Nissan or Infiniti dealership and buy part number 31086-CD100
Some of the early G35 only had that cap without a dipstick, it's like $20 for the dipstick, alternately you can go to a pick-n-pull and take one off a 2003-2007 350Z with an automatic transmission. That Nissan part number is the 350Z dipstick which works in the tube that's equipped on your style of transmission.

 

You are correct that 'flushing' a trans can cause issues, these cars you do a drain/refill easily since the pan has a drain bolt. If you are mechanically inclined, I'd drain the fluid out and see how it looks. Its only 3.5-4qts. Heck, it might even have the wrong spec fluid in it, which is known to cause issues with these cars.
I personally would use Idemitsu Type-J or OEM Matic-J (some online vendors still have it in stock). I pick them over the Matic-S, especially if you have slipping issues. From what I've heard, they apparently they changed the additive package with Matic-S to make shifts softer for the 7AT trans in newer G/Q's. My 04 G shifts more consistently with the J spec fluid compared to S.