Fluid level is spoken about often here are the facts. With and auto trans and in particular the 4L60E there is no such thing as low. If you let the trans get even 1/2 quart low you will and or have damaged your transmission. They are absolutely not forgiving. We recently did some intentional test with a unit we use to check different ideas. Here is what we found, At 1/2 quart low approximately 1/2 inch bellow pan rail we ran the transmission. We monitored pressure. The pressure in this condition was completely unstable. Jumping from very low to beyond the max of our 300 PSI gauge's. When you changed gears particularly on the 2-3 3-4 and the reverse gears this condition became worse. I could easily see how after doing these test you could do a lot of damage to your transmission even this small amount low. In the case of the low pressure the result would be obvious burned frictions however at the same time the pressure spikes could just as easily break parts, push out snap rings etc causing serious damage and or sudden failures.

Now in a past test several years ago I did a similar test going in the other direction too much fluid. Here is what happened there, this was however unlike the first test with a 4l60e it was a 700r4 though would imagine the results would be similar. Adding one quart over after hitting the full line did not seem to have any notable effect. However adding two quarts over the trans began again to have pressures resembling the low fluid scenario the pressure dropped and spiked erratically and of course the fluid began quickly to resemble the appearance of the low fluid trans lots of air bubbles and since a trans is not so different from a brake system the result was similar. You cannot have stable hydraulic pressure with air in the fluid.

The air created in the low fluid scenario is a result of the filter becoming uncovered and the pump pulling air into the system. With the resulting crazy pressure readings and the extremely high fluid this occurred because the internal rotational assemblies in the transmission were acting a lot like a blender and churning air into the fluid more rapidly than it could bubble out.

The Point on this is fluid level is imperatively important with and auto. There is no such thing as a little low. A little low means you just took years off the life of your transmission or even killed it right now.

All transmission should always be kept at the top of the crosshatch level warm running in park or neutral. Ideally I wish there was a way to assure the transmission fluid was equal to the pan rail level in gear running . But I do understand why they don't have you checking it that way

Another thing that should be addressed is aftermarket and stock deep pans. Some aftermarket pans while deep do not require a deep filter. Others require and extension. Still others use the deep filter like goes in the truck 4L60E. Always carefully read the pan manufacturers instructions. While regardless of the pan type the dip stick will still read the level correctly. If you for instance use a deep pan with the wrong filter bear in mind GM actually used the bottom of the pan to hold the filter up in place. True the little seal feels tight holding it in the pump but it can work its way out and drop into the pan when used this way. This will result in the transmission acting like its low on fluid even when its not. At the same time if you place a filter that's too tall for your pan combination it will crack in the neck area and again cause the symptom of low fluid. I thought I would mention this because we have run into this with some of our own customers changing pans and not being aware of the filter differences and requirements. So always make sure you are using the correct type filter for the pan you are installing. Also note when installing a transmission. If you do not provide even support across the pan again you can bend the pan and damage the filter causing the same set of symptoms.