what motor oil should I use for my 82 750 Seca?

I have just resurrected an 82 Xj750 engine after sitting in a crate for 27 years, the book says to use 20/40 oil but all my knee dragging friends say I can use 10/40. is there a difference what are the pros and cons?

 

Your signature doesn't have a location, but unless you're riding in cold weather, 20W50 is probably a better choice than 10W40. Personally, I use 20W40 except in the middle of summer when I will use 20W50 (20W40 IS still available you just have to look for it.)

The biggest caveat is to stay AWAY from the new generation of AUTOMOTIVE oils that say "fuel saving" or the like. They contain friction modifiers that cause clutch and starter problems. Personally I use specifically "motorcycle oil" but you can use "normal" oil, just make sure it's really normal. Synthetics, unless specifically formulated for motorcycles, are a no-no for the same reasons the friction-modified oils.

If you'd like to read up on a lively debate on the subject, see: http://xjbikes.com/Forums/viewtopic/t=790.html

 

Thanks for the reply, that helps a lot; I am in North-Eastern Washington, mostly cool-mild weather, except for the month of August when we break into triple digits.

 

You should be fine with 20W40 in the summer, I'd only move to the 10/40 if you get down to the frosty side.
I'm in southern CA and run 20W50 year 'round as it never gets cold enough to warrant thinner oil.