Oil Grades
Firstly, the Lycoming Engine was designed many years ago when multi-grade oils were not generally available and a certain amount of their design was dependant on the best oil available at the time. I am personally drawn to this fact and would sooner use what was recommended by the manufacturer at the time. Lycoming doesn't commit itself entirely to any, and simply recommends the use of a particular grade. Let it be known that there is no definite approval for engine oils, only that they have to meet a certain specification. You didn't specify the engine model and this also has a factor. The Lycoming TIO-360 would be better left on 100 grade oil, whereas a Lycoming O-235 G or H would be better on 80 all year round regardless. However, when all said and done, the aircraft manufacturer's recommendation is the over-riding factor and not Lycoming. This is because of the design of the aircraft, the engine installation, the cowling, the oil cooler system and so on. Read your flight manual or operators handbook or consult the aircraft manufacturers literature or type certificate. One thing is for sure, spend the money and add the Lycoming oil additive at every oil change even if is not a mandatory requirement. Note that legally speaking, you can't make it up as you go along, the aircraft manufacturer's recommendation is your first option and then Lycoming's, unless you wish to apply for a minor modification, approval or dispensation to use any other.
Rgs. Bob.