Thickest oil grade you have run in winter, summer?

10W-30 oil stopped a 2003 Mazda Tribute V-6 dead in its tracks at -29C years ago.

Later, I had Mobil 1 10W-30 in a 2010 Kia Soul, and I thought it would be ‘okay’ because it was a full-syn 10W-30. It did the exact same thing, would not start below -25C.

I don’t use 10W-30 in the cold part of the winter anymore. I don’t know how people got going in the 1970’s when 10W-30 was the ‘winter’ grade of oil.
 
Straight 50W Vavoline VR1 in my brothers car to keep timing chain rattle at bay. Although last oil change I added a quart of 20W 50 VR1 along with the 5 quarts of 50W I had on hand. 20W 50 VR1 is all the local auto parts carry.
 
10W-30 oil stopped a 2003 Mazda Tribute V-6 dead in its tracks at -29C years ago.

Later, I had Mobil 1 10W-30 in a 2010 Kia Soul, and I thought it would be ‘okay’ because it was a full-syn 10W-30. It did the exact same thing, would not start below -25C.

I don’t use 10W-30 in the cold part of the winter anymore. I don’t know how people got going in the 1970’s when 10W-30 was the ‘winter’ grade of oil.
Kerosene or MMO doping would be my guess.
 
10W-30 oil stopped a 2003 Mazda Tribute V-6 dead in its tracks at -29C years ago.

Later, I had Mobil 1 10W-30 in a 2010 Kia Soul, and I thought it would be ‘okay’ because it was a full-syn 10W-30. It did the exact same thing, would not start below -25C.

I don’t use 10W-30 in the cold part of the winter anymore. I don’t know how people got going in the 1970’s when 10W-30 was the ‘winter’ grade of oil.
Back then we didn't have sissy starters and batteries like these kids today
 
I've never been necessarily brand loyal, but I used a lot of Quaker State and Havoline conventional 20w-50 on and off from the early 90s to the early 2000s. I used it year round in my 1987 Oldsmobile Delta 88. Thinnest oil I've ever used personally was 5w30 in various cars, again year round. I think 15w40 is about the perfect compromise for most engines in most operating conditions, but that's just me.
Try that up here in the snow belt; doubt it works very well
 
in 80s VW rabbits, 20W50. Ran that year round.

This was mostly Castrol 20W50 and some Texaco Havoline. Whatever was on sale at the local KMart clones...
Caldor.. Bradlees.. etc. I am dating myself with those references.
I also had a 1977 Rabbit that I ran Castrol 10W50 year round. The 10W50 was hard to find but I really had no issues with it back then, lived in CT at the time.
 
Kerosene or MMO doping would be my guess.
Cars were hard to crank AND start. As Teens were driving mid sixties cars then. I recall having issues with carb freezing too.
Of course the choke stove and intake crossover was all carboned up, and the single-snorkel factory air cleaner housing was replaced with a Cal-Custom open element shiny chromed part :)
 
Back in the day I ran Castrol HD-30 year around here in Michigan. Winter. Summer. Didn’t matter. Did that for years with zero issues.
That is still what I use in my lawn mower. But in my truck I use Castrol Syntec 5w-30.
 
Dad said in his 67’ CUDA with 273 he ran Pennzoil SAE 40 for longest time. It touted “with Z-7” on cardboard can
 
10W-30 oil stopped a 2003 Mazda Tribute V-6 dead in its tracks at -29C years ago.

Later, I had Mobil 1 10W-30 in a 2010 Kia Soul, and I thought it would be ‘okay’ because it was a full-syn 10W-30. It did the exact same thing, would not start below -25C.

I don’t use 10W-30 in the cold part of the winter anymore. I don’t know how people got going in the 1970’s when 10W-30 was the ‘winter’ grade of oil.

Block heater’s and leaving it running all night long.
Things were simpler back then.
 
Back when I was 16 and knew nothing about motor oil, I ran 20W-50 in a 1965 Ford 289 V8 all year long in the midwest. After it barely started a few times in below 0F winter weather I put a block heater on the car which made it easier to start. I'm sure that cold thick oil wasn't good for the engine, but I never saw any negative effects. Obviously, today I'd be running a 0W or 5W in the same situation.
 
15W40 year round in the 240D and still doing it in the 250D. -5C is about the coldest it will ever see.
 
15w-50 in summer was thickest ( tough Now I do 5w-40 or 0w-40) .
Back in the day when I had my VW passant it called for 15w-40 as winter oil in the early 90s to mid 90s.
(and 20w-50 as summer oil)
Part of the reason is back then 5w-40 oils were still a bit of a rarity.
A few short years later the recommendation had changed to 5w-40 winter oil as more "legit" 5w-40s hit the market.

VW engines were well known for hot spots so they wanted the extra oxidation resistance.
 
Not counting HDEOs in my diesel truck, 5w-40 in the winter in my 3.5L EcoBoost (Gen2) F150 and 10w-40 in the summer in a GMC Savana 3500 box van with the 5.7L V8.
 
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