Yes, it's very heavy, which is why it slowed your leak, which is also why it didn't improve your mpg, which is physically impossible by going up in viscosity.
I would suggest a proper mechanical fix for your leak.
If you want to keep diluting your oil's additive package with heavy bright stock, see if you can find some wholesale, the price is super cheap:
Even at the "high" price of $0.0776/gallon, it's massively cheaper than Lucas if you can find a source.
The referenced BS150 product is 28-33.5cSt @100C, which is off the chart for SAE J300, which tops out at 26.1cSt for an SAE 60:
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