Too much toner on paper which then smudges

None. It is electrostatic attraction.
It varies from each type of laser printer.
Most printers will specify a total number of pages printed from a laser toner cartridge rather than give an actual amount of toner per page. Which I feel would be more likely to be an average value.
Due to the amount of toner being used per page varying on the number of actual pixels that are required - a black dot or a color dot. If a colour dot is required Three toner colors may be involved to get the color required. Where there is a white dot no toner is required.
It really depends on what image is being rendered onto the paper on each different page image print onto the paper.
Color photos use more toner as more of the page is color than when using black and white !

try to stop the sheet just before it enters the fuser and see if the marks are on the paper already. if not then it has to be the fuser if they are them you have another issue.

this is possibly a problem in the fuseing part of your machine, where the paper heats up at the back, the fuser roller could be worn out. and your toner could be sticking to it.

Problem is either the cartridge or the fuser.
Put paper in the manual feed tray(tray 1)
Print something. When the paper gets 1/2 way into the printer, open the
top cover. Pull out the toner, look down at the paper.
If it has the smudge replace the toner. If no smudge, replace the fuser.

If this just started after you replaced the toner cartridge, one would have to say that it is a bad toner cartridge.
Print something that smudges. Pull down the front cover and look at the cartridges. Pay attention to the top one(black).
Look at the drum on that cartridge. You should see the toner sitting there. Compare it to the other cartridges which should be clean.

How do I clean white or black spots on printed documents? Description: First make 10 to 15 copies of a blank, white sheet of paper.
If you still see the spots on printed documents after making multiple copies, clean the drum unit as follows:

1. Remove the drum unit and toner cartridge assembly, and then remove the toner cartridge from the drum unit.
   
2. Position the print sample in front of the drum unit so you will know the exact position on the drum that is causing the poor print.
   

3. Turn the black gear on the drum as you check the surface of the OPC drum.
   The OPC drum is photosensitive so do not touch it with your fingers.
   

4. When you find the substance on the drum surface that matches the print sample, gently dab the surface of the OPC drum with a cotton swab that is lightly dampened with isopropyl alcohol.
   
Note
• Do not use a scrubbing or wiping motion because this might damage the drum.
  
• Do not try to clean the surface of the OPC drum with a sharp object, such as a ball-point pen, because this might cause permanent damage to the drum unit.
5. If the print quality still has not improved you need to install a new drum.
   

This is the test to determine toner/drum or fuser issue.
Print a test page
When you hear the paper feed into the machine open the top to stop the machine
Remove the toner/drum cart.
You should be able to see the page with printing on it. (This is before the page hits the fuser)
If you can see the smudge or marks on the page the problem is most likely the cartridge but NOT the fuser.
If the page is clean the problem is most likely the fuser.

Sounds like a fusing error ... If your able to wipe the toner off the page its a fuser unit problem (replace fuser unit) If toner is sticking to paper but the print quality is poor , replace the drum unit  (PCU)  good luck

Hmm... This sounds like it is a toner problem. If your roller is getting too much toner then it just globs it onto the paper even though the drum is not asking for it. Are you using a new toner cartridge or a re-filled one? Is it from Brother or a 3rd party? I suggest you remove the toner cartridge and check inside the printer. Clean any spilled toner. Check the toner cartridge for leaks .. give it a good shake and see if it is leaking. If you have another toner cartridge, you can replace it to test. You may need to pint a few pages to see a change. Another test could be to print a few (2-5) "BLANK" pages. The printer should not be depositing any toner. If no smudge or the smudge stays the same, then the toner cartridge is the likely problem. If the smudge is there, but gets smaller, then clean your printer.

Good luck!

Can only be two things the drum or the fuser. More than likely drum they last about 4 toners i think.

Learn the art of getting a page to stop halfway threw printing, you want to stop the page after the drum has put a image on the paper but before paper has gone threw the fuser (hotbit).

Once you have mastered the position correctly by stopping the printing by opening the door and removing drum/toner cartridge, to reveal the paper that is printed but can be rubbed off, we will know if marks are coming from the drum (you will see them) or from the fuser, if print is fine.