New toy for the garage: needs a full service.

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1990 Honda 300. A brief look over the honda forums seem to favor 5w40 hdeo and 0w40 powersports oil, and basically any 80w90 gear oil for the rear axle. Am I on the right track? I also need to figure out something on the air filter, it's a little dirty but the foam is a little small for the filter.
 
Yeah, I would say almost any oil in the 5-10-15w40 range would work well in the old Honda. My brother has two 80s Honda quads, he runs GTX 10w40 with fine results. Just be sure to change it fairly often, the transmission gears won't be kind to the oil's viscosity. Gear oil, you're spot on. As for the air filter, I would recommend buying a new UNI filter. The foam gets hard and brittle with age, be safe and replace.
 
So I went and looked to see what I had on hand. I have 2 qts of Amsoil small engine 10w30/sae30 and a gallon of delvac 15w40. Also found a qt of 75w90 severe gear and some Amsoil Quickshot for the gas.
 
I've run the Amsoil small engine oil in an air cooled Suzuki before, that is a very stout oil, it would work well. The Delvac will also work well, maybe it for the summer and the Amsoil for winter riding. I don't like Quickshot, but if you have it, it won't hurt anything, I say use it, maybe it will clean the carb up some.
 
Well I decided to use some T6 since I have the other oil for other things. I guess the main question now is about the seafoam/engine flush. Also, capacity listings online list a gearcase capacity of 6.4 oz. I'm assuming that's for 4wd models ?
 
I say go for the engine flush you have, don't leave it in long, change the filter and then run a 10 hours oci then change it again. I would assume on an old quad that it wasn't maintained too well over the years, lots of gunk to remove from the engine. You'll want to change the oil more frequently until the color doesn't go black too quickly.
 
Bottle says to idle 15 min then drain, im sure it wont take the whole bottle for that little engine. Hopefullly it won't harm the clutches.
 
I wouldn't use the Sea Foam in the engine of that Honda. If it doesn't smoke it will. Don't do it. Use it in the fuel. Its Great stabilizer. Just change the oil. It will be fine. That comes from the owner of many fine Honda ATCs poorly maintained by a tennager. Use any oil and change the filter and it will run forever.
 
I have a 2000 model trx300 same bike as yours really , I run t6 5w40 or delo 15w40 . never had any trouble . These are very durable little quads . A new air filter doesn't cost much from a Honda dealer or off the net . I wouldn't run the seafoam in the engine oil , maybe the fuel . Good machine ya got there in my opinion and parts are reasonable priced if you have to buy any .
 
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