20-50 mineral oil and viscosity

I have used 2-3 brands over the last 35 years, but recently bought a box of 4 cans of Gulf 20-50 mineral oil with a higher zinc content.
I have changed oil on 2 cars , and cannot understand how the Gulf oil can be a 20-50 oil, as it pours so easily. It feels like a modern 5-30 oil.
My previous oils took twice as long to fill, as it took so much longer to find its way down the narrow passages in the block.
Has anyone else tried the Gulf oil?

First, does it say SAE on it? Is there a W after the 20?
If not, I wouldn’t trust it.
If it’s SAE 20W-50, then you could run a viscosity test, like SAE does.
Find a very small funnel, and pour a liter through it, timing it.
Same test for two or three known viscosity oils, and compare the times.
When SAE does this, the test uses a specific hot temperature and a very carefully calibrated orifice, and the time is known as the Saybolt Universal Viscosity Seconds.
Times within certain ranges are given their SAE numbers without the W . The same test run at a cold temperature gets the W number for Winter.

Gulf was once a major brand of gasoline in the USA.

They sponsored the Porsche 917s at LeMans around 1970.

Good point Rob.
Just checked and it says 20W-50 mineral oil

By “mineral oil” do you mean non-synthetic? Although motor oil, synthetic or not, is indeed mineral oil, the term is usually (in the US at least) reserved for highly purified light oils that are clear in colour. People drink mineral oil, but it’s not recommended to drink motor oil AFAIK.