Jim- The only ratings for the current lubricants sold for most lower unit applications are API MT-1 and GL-5, with the exception of some special synthetic (silicone based) ones for extreme high -performance (racing) uses and some formulated for the older OMC electric shift gear cases, according to a product availability survey I'm making for you through several catalogues.
Most manufactures market their own brands of lubricants and other service products to profit from this high gain market. Unless someone tells you different with comensurate authority and the accompanyng reasoning, I would think it would be smart to go ahead and find something more reasonably priced.
Most gear lube is priced at either $4 or $5 for a 10 oz tube (convenient for filling from the bottom of the gearcase), which works out to $51.20 or $64 per gallon, respectively(!!!) For about a dollar more than the 10 oz tube, they'll sell you a quart bottle instead, hense your $25 a gallon figure.
One might be tempted to use lubricants intended for cars. I doubt, however, that regular 90 wt hypoid oil for automobile rear ends has the additives for marine use.
A less expensive alternative to you, since you live in Antioch, Ca., would be to drive to the Bargain Barn in the rear of the West Marine Store in Oakland. They have 10 oz tubes of Lubrimatic Lower Unit Gear Lube marked $1.59 each that were sold to me for $0.77 each (marked down at the cash register as "distressed" if you humor the friendly clerk), which gives you a final price of $9.86 per gallon. Hope this helps. Joe
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