I have an 85 E350 chassis RV with a Dana 70 4.10 rear end. It has potentially been 25-30 years since the oil was changed, if it was EVER changed at all. I want to change it a few times with cheap conventional gear oil to help clean it up before I put in more expensive full synthetic.
I have my eye on a $15 gallon jug of SuperTech 80w140 for a short change interval of maybe 500 miles, but it's only spec'd to API-GL5. Would that be acceptable?
I haven't opened it up yet so I don't know if it's an open or limited slip diff. Adding a $10 bottle of Motorcraft friction modifier still makes it only $25 total.
I plan the "final" fill to be Valvoline full synthetic 75w140 which is over $50 for 4 qts, so obviously I don't want to use that for the short term use cleaning process.
I ordered a $22 "Lube Locker" rubber differential cover gasket so I can easily pull it back off multiple times in a relatively short amount of time for changing the oil. No RTV required, so I will only have to scrape the surfaces of RTV/gasket the first time.
If it's an open diff, I might even just stick with the cheaper $15/gal conventional instead and change it every 12 months since the rubber gasket makes it so easy.
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