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I would think that the 75W-90 gear oil was the savior. Actually you might have reduced the protection afforded by the 75w-90 lube with the addition of the stp.Though, there are things you can find in a can that might delay the onset of large mechanical bills.
In 1977, I bought a used 1974 Mercury (Ford) Capri 2-liter. After a few months, I figured out that the clutch was toast, so I took it to a local mechanic to get the clutch replaced. Not too long after that work was complete, I started hearing a loud noise coming from the transmission. Long story shortened. My local mechanic had drained the transmission before he removed it to gain access to the clutch and forgotten to refill the transmission when the work was complete. I filled it with a 50/50 mix of 75W-90 gear oil and STP Oil Treatment. The transmission was now quiet and I drove that car for about 6 months before trading it in on a new Toyota Celica. "Mechanic in a can" worked for me!