My mechanic recommended I use 10w -30 since the car has high lot of miles because in short it’s thicker & for such for such a tripThe engine runs, the torque converter spins, the transmission scavenge and pressure pumps operate. Almost all the fluid in the pan is pumped up into the side tank - which is where the dipstick is. So, with the engine running in P on a level surface you measure fluid level. The "no fluid on the dipstick with the engine off" is a caution against overfilling.
There is no need to disconnect the top radiator hose to get to the transmission dipstick.
Your engine should easily get 2,000 - 4,000 miles to a quart of 5W-30 oil. Mine gets over 6,000. You probably need to aggressively exercise the engine out in the boondocks.
Study this - all of it. Written by a CF member who was a GM Northstar Systems powertrain engineer. It was written before the 2000 changeover to roller cams, so some of it is obsolete, as is the section on the discontinued use of cooling system sealant tablets. Read the WOT section - "Occasional full throttle operation is good ..."
This is a Seville engine compartment, the air intake system and surge tank is a bit different, but the parts ID is the same as your Deville. Which model - base, DHS, DTS ? It makes a difference in advice. You NEED to subscribe the car to the online GM/Cadillac service manuals - www.alldatadiy.com
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