You have either a blown head gasket or a cracked head. To determine if the head is cracked requires taking it off the engine and taking it to a machine shop and having it pressure checked. In an extreme case you could have an internal crack in the block but I would suspect the gasket and start there.
Any type of antifreeze will work in a 99 Saturn SL1. Many people recommend using Prestone Antifreeze for cars that are older than ten years.
5w30
5W-30
On the 1995 Saturn SL1 The oil pressure sending unit is the sensor closest to the alternator. The unit is on the back of the block.
Autozone/ advanced auto usually has antifreeze that goes with either kind. I suggest that.
into the coolant reservoir on the passenger side of the engine bay by removing the cap
up a mountain
Yes it is.
The recommended Oil for a manual transmission in a 1997 Saturn SL1 is synthetic multi-vehicle automatic transmission fluid or ATFQT. The recommended engine oil is SAE 5W-30 synthetic motor oil.
on my 94 it was near the oil filter and it was a canister type like your oil filter
If you truly like your Saturn use a NAPA gold filter FIL1348
Remove transmission oil pan