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Here is the oil fill procedure I use today to address the whole overfill conundrum, use it at your own risk as I am not a professional automotive technician. If you have any concerns about aspects of the procedure contact a professional for their advice.

• Park the car on level ground and break-free and leave in place the passenger side rear wheel nuts slightly snugged.
• Put the rear of the car on jack stands, Jacking-up a 993 a DIY procedure to do this safely;
• If your car still has the under engine cover/tray remove it and store it away as some in this community think it just hides oil leaks and causes the engine to retain heat. I feel it serves little purpose beyond making changing the oil more difficult. It has gone missing on most examples of a 993.
• With the passenger rear wheel now removed, I drain the oil tank and the engine case into a drain pan of at least 10 to 12 quarts in capacity. Do not attempt to drain the oil using a vacuum oil remover through the oil filler neck, it will not work and can easily damage or bend the oil level float arm in the oil tank.
• The engine case drain plug is on the bottom of the engine case and points to the driver side of the car.
  • I reach under the car through the passenger side rear wheel well, I replace both filters. This requires removing the fender liner, the back half of the plastic rocker panel cover and detaching the heating system flapper valve rubber flex duct on the passenger underside of the engine case.
  • I have used both a channel-locks or an oil filter pliers to initially break loose both the small and large filters.
  • Use a little old oil on the rubber seal on the new filter before threading it in place.
  • I make both filters as tight as I can with my bare hands and then turn the filter a quarter turn beyond this with the pliers. Although not a factory method I have never encountered a filter backing off or being unable to remove one with this method.
  • I do not disconnect any steel tubes as specified in the factory procedure to drain the oil in the oil lines as my belief is that breaking the seals presents unnecessary leak failure modes and the quantity of oil present in the lines is small.
  • I do keep a can of spray electric motor/electronic parts cleaner on hand to clean off oil spilled on the engine and suspension when removing the smaller filter.
• When replacing the engine case and oil tank plugs I replace the crush rings and the "O" ring on the tank plug. On occasion, I have reused the existing metal crush rings and "O" ring with no leaks. Be sure not to overtighten the tank plug.
• Button up the plastic parts, the heating duct, mount the wheel lightly snugging the wheel nuts and put the car back on the ground
• Retorque the wheel nuts to the recommended dry thread value of 133Nm. or 96 ft/Lb in a star pattern sequence, then check the torque values by repeating the sequence on the now tightened wheel nuts.
• Refill the oil tank with 8 quarts +/- of oil and run the engine to a fully HOT state, this takes a while, looking to the temperature and oil pressure gauges to see if they have settled into their normal operating range.
• I then, with the engine running, add additional oil, ¼ quart (8 oz.) at a time until the dip stick is at approximately 1/3 of its scale with the car on level ground with a fully HOT RUNNING engine.
• If you find yourself putting ten or more quarts of oil in, using this procedure, you are most likely overfilling the engine. On a 993 this can have serious operating consequences and so an overfill should be addressed as soon as possible. The easiest way to address an overfill is to shut off the engine and drain the oil from the engine case as the case does not retain much oil.

I feel this method gives an additional margin of safety in avoiding inadvertently overfilling the system. If you decide to follow my measurement procedure and as a follow-up check the fill with the dash gauge and your gauge doesn’t read at about 1/3 of its range the float in the oil tank is a bit out of adjustment.
Adjusting the float so the gauge approximately matches the dip stick involves draining the oil tank accessing the oil float in the tank and bending its arm a little. I have done this on my 86’ Carrera and have been told it is not too difficult on the 993. If you want to make the adjustment it is something to consider at your next oil change when the tank is drained.