...you probably won't need to use sysnthetic or synthetic blend oil. When Chevron Supreme, Havoline, Pennzoil Purebase 10W-30 SL oils are available for under a $, in some cases less than 0.60, you'll find a case will handle 2 X 6 quart changes.
Oil filters are often on sale at 2 for $5-6. I don't especially like Fram but some of the other inexpensive ones are really very good: WIX, NAPA Gold, Pennzoil PZ-33, Purolater, etc. The WalMart SuperTech filters are very highly regarded by some, and they are cheap.
Say $6 for oil and $3 for filter and you can change it 3 times for under $30.
The nice thing about a quickie-lube joint is that they actually do get underneath and check the levels in your diff's, etc. If you know who you are dealing with ("Did you want fries with that...?") that can be convenient.
There is no question that synthetic oil is superior for longer intervals, in some cases substantially longer. MOBIL 1 is great oil! IMHO It depends on how many miles you actually drive in a year: AMSOIL AMO 10W-40 or AMT 10W-30 and a filter change and top-off every 6K miles should easily last for 18K miles per
year. Say 8 quarts @ $5.70 + 3 filters at $3 + $54.60
And that's not a lot different costwize from 6 conventional D.I.Y.changes each 3K. Just less time getting dirty.