Oil/oil Change

Synthetic versus conventional oil, ok, with a limited amount research I've done it has been said by THE people that research and develop motor oil say this, you can switch back and forth between synthetic and conventional oils, it's not recomended, but you can do it. You can also throw a quart of conventional oil in when you are low and don't have any synthetic available. As for it leaking in an older engine, well it was probably leaking in the first place, it will not make it worse per say, some brands claim to clean the engine's inside so in that case it will leak more because it has 'cleaned' the space where the leak is.
Here's my advice, no matter what oil you use, don't switch between brands every time you change your oil, some don't play well with others, buy a quality oil filter, every time you change your oil, change your filter, I recomend NOT to use fram, wix is actually a very good filter that gets a bad rap.
Example: 1997 SD 7.3 turbo 540.000 + miles. Same wolfs head semi synthetic truck oil every 6 to 7k, even 8k a couple times never had a problem, it will use about 3 quarts in that time, it holds 14 quarts, only rear main once, it does need the front crank seal now but does not leave but a drop or two overnight.
But thats what I know, take it for what its worth.