What oil type should I use on a 68 ford galaxie xl?

I went to the auto store to ask but it wouldn't come up on his computer. He suggested 10w-30 so I bought a jug, but I want to double check before I put it in my car.

If the engine has low miles then SAE 10w-30 is a good choice. If it has high mileage then use SAE 10w-40.

Yes, the weight of 10w-30 should be perfect for your classic Ford.

10-30 should work fine

Your engine has a flat tappet cam shaft. The oil your car needs is harder to find today. It isn't just the viscosity, 10/40 OR 20/50… But ZDPF ADDATIVE that is needed.
classic car and hotrod oil!
summitracing.com
LUCAS OIL CO…"CLASSIC CAR AND HOTROD OIL" PART# 10684-1 THIS IS A 20/50 OIL PERFGECT FOR AN AGING 60'S ENGINE
YOUR OIL NEEDS ZINC TO PROTECT THE CAM.
THEY TOOK THIS OUT OF OIL TO PROTECT CATYLITIC CONVERTERS…

You need to be concerned about the ZDDP levels in the oil with flat lifter cams…
Racing oils do, but they have little or on anti corrosive additives… There are some specialty oils. I personally like 0W-40 Mobil 1, available at Walmart in 5 qt bottles for under $25. Here's some info from Mobil with other possibilities http://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/ZDDP_Levels_Classic_Cars.aspx

10-30 should be just fine…

10-30 is what you should use all year round.

This falls under the jurisdiction of API. According to them, any modern motor oil will work. If you're concerned with the lower levels of supplementary additives found in today's motor oils, you can always add performance additives such as ZDDP or Molybdenum compounds sold as anti-wear engine oil additives. Or you can purchase modern high performance motor oils which contain advanced additives. Here is a link to some informal API information regarding this topic. http://www.api.org/certification-programs/engine-oil-diesel-exhaust-fluid/~/media/Files/Certification/Engine-Oil-Diesel/Publications/MOM_GUIDE_ENGLISH_2013.pdf

If your engine has just been rebuilt then any oil of the right weight is ok. If you have lots of miles on it there's a danger that the detergent additives in modern oils will dislodge stuckon crud and varnish in the oil ways and cause blockages. It's best to use a very inexpensive low tech oil unless you block has been dipped in a hot tank degreaser during a recent rebuild. So I'd steer away from ultramodern synthetics etc.