blew my efficiency

So, I blew my efficiency on an oatmeal stout today. My OG was about 1.028... Only thing I can figure is I wasn't watching my sparge temperatures closely, but I know for a fact that the mash held at a solid 150 degrees for 90 minutes...

So not knowing what else to do, I made it up with about 2.5# of extra light DME I had laying around. There's no way I wanted to dump half a day's work. When I added that I was on target for OG... Could there be any issues resulting from this in the final product?

Please post recipe and your steps that you took. ALso include type of sparge and timing.

7.5# Marris Otter
0.5# Choc Malt
0.5# 55L Crystal Malt
0.25# Roasted Barley
1# Flaked Oats

Mashed at 150 degrees for 90 minutes--about 3 Gallons of hot water. Cooler held temp well. Sparged twice with 185 - 178 degree sparge water. I didn't watch the sparging temps in the cooler well, so I'm wondering if that's where the issue came from. My sparging consists of siphoning water from sparge pot through cooler valve, and then draining into wort pot--whatever type of sparging that is...

Start boil volume was around 7.5 gallons. Boiled down to 5 gallons with:
1 oz East Kent Goldings
1 oz Fuggles

Batch or fly sparge?

My sparging consists of siphoning water from sparge pot through cooler valve, and then draining into wort pot--whatever type of sparging that is...

Next time try completely draning your mash tun, then adding half of your sparge water and stir the grains. Let it sit for about 15 minutes and drain completely. Do a 3rd time with the remaining sparge water. That should improve your efficiency.

As far as the beer, if there was enough sugars taken from the specialty grains and oats, you should be ok. Those will drive the flavor profile more than the base malt in this situation.

What do you have in your mash tun for lautering? False bottom, stainless braid, other?

It sounds like you were fly sparging. How long did that step take?

What efficiency did you get with the other two AG brews?
If your efficiency has dropped for this batch, did you change anything in equipment, or procedures?
When did you measure the gravity. Pre boil or post boil?
If pre boil, what was the temperature, and did you compensate? and most imporantly did you stir really well before taking the sample?

-a.