1994 Chevy cavailer with speed issues

Hello I am driving a manual transmission 1994 chevy with about 225000 miles. About a week ago I did a oil change on my car, but instead of using 10w40 is used 5w30 as "indicated" by my oil cap. I drove it for about 3 days and noticed that when starting in first gear I put force on the gas pedal but I dont go very fast. When I try to reach higher RPM's my engine shakes, rather violently. It does the same thing in 2, 3, 4, and 5 gears. I changed the oil, putting in 10w40, and the car ran a little bit better, but still basically the same thing. I notice when I play with the clutch while accelerating I can improve speeds, but still I get that terrible shaking. There have been no stalling problems, nor startup problems, just when I drive it usually goes 1st gear up til 10 mph, then 2nd gear up til 20 mph, then 3rd gear up until hopefully around 35 mph, and 4th gear till about 45 mph. In 5th gear it requires almost pedal to the floor to accelerate. My thought is that something is either off with my timing or a tune-up is needed.
also, if this helps at all, yesterday on my way home from work I was able to get my rpm's in first gear up really high really fast by playing with the clutch, and after I shifted into second gear everything seemed like normal! But unfortunatly as soon as I stopped to make a turn at a light, everything went back to normal (bad normal). Im really concerned because ive been flying through a weeks worth of gas in like 2 days, and ive been skeptical about going to a mechanic because its a old piece of crap that they might try to hit me up for parts I dont need.
any input is certainly appreciated! Thanks!
~mike