Different mineral 20-50 oils

I have used the same oil for 20 yrs and been very pleased (Swedish private brand but made by the same refinery as Esso used for their, more expensive products).
Last year I began using Gulf’s MAX and Classic. Almost identical but the metal can Classic has more zinc. Twice the price to my previous oil.
What I cannot understand is the perceived difference in viscosity. Both are 20-50 mineral but the old oil is like ”molasses” and the Gulf oil is like a synthetic 10-30 oil, i.e. It pours like ”water” in comparison.
This is very evident wheh filling the XK engine, as it takes time for the oil to make its way diwn the engine. The old oil took twice as long to fill compared to the Gulf oils.
Can anybody explain this phenomenon???