Lets not get into the pro's and con's of synthetic oil but my question is: I just realized that Mobil comes in many weights, 0-20W, 10-30W, 5-30W any ideas with an engine with 80K what weight would work the best. Climate is new york state.

I'd use what GM recommended. 10w30. I used that in my '92 4.9 til 125K when I traded it and never had a problem.

ok...so what oil should i use...

'94 concours deville...133,grand.....

recently using 5w-30 synthetic, until i realized that using natural mineral would be better

if im right 5w30 is thinner than 10w30 and would be a good choice for winter time with the cold weather but there isnt really too much of a difference with 5 and 10. i mix then all the time if i need a top off and cant remember what i used. havent had any problems.