Am I wasting my money?

[SR5]

My mom has a 04 Marquis with 150K miles on it that sits up in a garage in high humidity. I use Castrol Magnatec because it is cheap and a synthetic. Could easily run Chevron supreme (Good and low cost conventional) Super tech Syn, QSUD, or Havoline but my dad LOVED Castrol GTX. That's a brand my mom has seen through the years so she is comfortable with me dumping that in the engine.

Yep GTX is an old name that many non-oil people recognise, it’s the same here.
For those people, who know GTX and ask me about which oil to get, I mostly suggest they step it up to Castrol Magnatec synthetic. A good oil, in a brand they know, that doesn’t break the bank.

 

[ARCOgraphite]

.... Oil change time is at hand. No engine problems and I'll have to add a quart halfway between changes which are every 7k

How long does it take you to get to almost a year? If SO I would run a good 5W20 and stick with it - and Motorcraft is a good oil as is Quaker State 5W20. The Edge is unnecessary. I Might change Spring and fall. The short tips are hard on the oil even though you do an 80 mile jaunt on Saturday. Is that 80 round or one way?

I would concern myself more with brake fluid and lines, water pump, accessory belts, EVAP system, and of course the transmission and transaxle.

Sound like you have been doing the right thing, so why change.

 

[ARCOgraphite]

...
For those people, who know GTX and ask me about which oil to get, I mostly suggest they step it up to Castrol Magnatec synthetic. A

Stateside at least Magnatec is marketed as a GTX variant. I wish the HM bottles werent so close in colour! I purchased a HM instead of magnatec once, but used it anyway - it was just for top-off

 

[JC1]

Since you are in Atlanta, I would just use the cheapest Conventional Super Tech or whatever else you can get easily. As others have already mentioned ST is a good oil. Do you buy oil on sale/promotion, or you only buy oil when you need it?

You could also get the Kirkland Synthetic if you are Costco member. I think you can get 2, 5 quart jugs for $22 when on sale. That's an even better price than the Supertech.

 

[ARCOgraphite]

Given the number of annual oil changes with a modestly driven car, cost is not an issue for anyone; Poor or Rich.
I would stick with one brand though. Of course if you go through a formula change with your favourite oil, you are at the mercy of the new package.