I have lots of oil /brands on the shelf and since my My Buick PA hit 100k miles and am going to experiment with blending my oil with:
2 Litres of Castrol GTX High Mileage 10w-30
2.4 Litres of Castrol Edge Synthetic 10w-30
300 ml of Lubegard Bio engine treatment LUBEGARD BIO-TECH Engine Oil Protectant
Changed oil/filter today so far so great. Peppy and clean again. I was at 39% on oil life monitor, car consumed a half a litre at 2,500 miles. Lets see how my high mileage blend does.
The Castrol Synthetic does Meet GF-5 however the Castrol High Mileage Did not, only SN etc, not GF-5.
In my next change I will use half Mobil1 Synthetic and half Mobil 1 Super 2000 high mileage, both of those meet GF-5 and Lubegard bio engine treatment as well.
Anyone else tried a mixture like this?

I like to use 2000 grit sand in mine. It helps to re-score the cylinder walls and clean out the carbon buildup in the oil paths. Then I'll use about 5 quarts of junkyard oil and 5 quarts of kerosene to flush it all out.

Oh, sorry, got off the path. LOL.

No, I use regular oil. With normal oil changes the engine should come out pretty clean. Unless you have some bad rings, then mixing any oils isn't going to do any good.

Thats Pretty Funny Mike, Actually that 2000 grit sandpaper does work well at cleaning the bores. I used it on the transmission accumulator, when I did the TransGo Kit. Clean as a whistle and very smooth.
The main reason I am trying a partial high mileage mix are extras not found in the regular synthetic.
Superior detergency that fights deposit build up
Superior oil burn-off protection versus leading conventional oils
Seal conditioners to help reduce leaks
Advanced additives that minimize engine wear
BTW it is difficult to get specs from motor oils cold pour point. From what I have read, Mobil1 synthetics are one of the best with coverage down to -40 or better.
Right Now in winter, having oil that is not like jello is important!!
Along with my research came this post from Corvette Forum on oil testing and wear characteristics falex , or shear strength index on psi for different oils.
Quaker State’* New “Defy” Motor Oil – Lab Test and Wear Test Data - Corvette Forum
Suddenly Penzoil Ultra sounds very very good!

i have never cared for syntetics, because when you consider cost into the equation, and the fact the conventional oil is good enough its not worth the extra money. i have taken all my drivers to 2 to 300k miles and never had one wear related failure, and all have gotten the same or better mileage as new at the time i trade them in. all i do is conventional valoline and a delco filter, average cost about 22 bucks for both every 5k miles

i tried the high mileage Valvoline one time they had it on sale so i bought it for my 03 monte and my bonnie they both had around 130 at the time and both lost like 2 mpg and idled rough and ran like they had winter gas in them

Ya Justin many like yourself enjoy the cost savings & just use regular dino oil and change it with a good filter, with high mileage success!
In winter I am curious, do you lower the viscosity or stay with 10w-30, how does it start when its real cold out??

whatever the factory is, the w bodies were 5w-30 and the bonnie and yukon are 10w-30 i think, no i dont do anything different, but cant really say for sure someone else wouldnt need or have different results as they sit over night in my garage which is insulated and has a big concrete slab to average the temps. i dont run the heat unless i am out there but the overnight temps are often around ten deg warmer inside than outside. they do sit out side at work for about 10hrs for me and 8 for the the bonnie and dont seem any different, the only difference being the trans shifting higher while warming up

I've switched to full synthetic in most of my other daily drivers over the years, but I generally bought them at lower miles (60k or so). Mobil1 Extended Performance is usually my product of choice, as its rated for 1 year / 15,000 miles. I buy the "oil change specials" when on sale for less than $35. Other brand synthetics I still go 10,000 miles between changes. Can't hardly argue with that from a price and convenience standpoint. However my Buick I think I'll leave as-is with dino oil, considering I just bought it with 159k miles and it'* apparently had no engine problems so far on the conventional stuff.