Anyway, for the OP... This is from Bel-Rays website to help clarify weights of motor oil vs gear oil . "A 10W-40 motor oil’s viscosity is actually similar to the viscosity of 80W gear oil, and the viscosity of SAE 10W-30 is similar to 75W gear oil."
You should already have the "L" ones, they hold the radiator too. The holes are threaded from the factory. But who knows what happens in 25+ years to one.
Bracket to frame is a flange head bolt 6x1.0 x 10mm, bracket to fender is a pan head screw 6x1.0 x 16mm with a nylock nut.
I bought the cheapest ones I could find once too, threw them away and bought OEM.
If it won't loosen or tighten it has galled threads. Will have cut and replace for sure. The top is the pivot, the bottom holds the adjustment. I lost one at the dunes once and replaced with SAE common bolt. Has held forever.