As always results may very. Make sure you check the free travel of the valve once you remove the valve spring and to run a zinc additive like ZDDP. Oil no longer has a lot of zinc in it due to the EPA.
Additional ZDDP is not required, just a bunch of snake oil by the sellers of the additive. The only time it really becomes an issue is if you're running extremely high valve spring pressures, something you really won't run into with a street driven engine, and definetly not on a stock 235.
Just from the description you give, while I wouldn't rule out weak springs, sounds more like sticking valve guides. They're probably varnished up. If you're going to do the springs, I would advise removing the head and taking to the local machine shop. That way, they can check the valves and guides for any issue, plus boil the head in the hot tank to clean it out.
